splitSpan issues demo

gistfile1.txt

year-test.journal:
```
2015/02/01 first half
    expenses  $1
    assets

2015/07/01 second half
    expenses  $2
    assets

2016/02/01 first half
    expenses  $4
    assets

2016/07/01 second half
    expenses  $8
    assets

2017/02/01 first half
    expenses  $16
    assets

2017/07/01 second half
    expenses  $32
    assets
```

Year balances are good:
```
$ hledger balance -f year-test.journal -p yearly                                                                                                             ~/devel/haskell/darcs-get/hledger/examples
Balance changes in 2015/01/01-2017/12/31:

          || 2015  2016  2017 
==========++==================
 assets   ||  $-3  $-12  $-48 
 expenses ||   $3   $12   $48 
----------++------------------
          ||    0     0     0 
```

Note how first transaction in 2015 is not included. Note that this is old period expression, so this bug exsits in master:
```$ hledger balance -f year-test.journal -p 'every 2nd day of month'                                                                                       ~/devel/haskell/darcs-get/hledger/examples
Balance changes in 2015/07/02-2017/07/01:

          || 2015/07/02-2015/08/01  2015/08/02-2015/09/01  2015/09/02-2015/10/01  2015/10/02-2015/11/01  2015/11/02-2015/12/01  2015/12/02-2016/01/01  2016/01/02-2016/02/01  2016/02/02-2016/03/01  2016/03/02-2016/04/01  2016/04/02-2016/05/01  2016/05/02-2016/06/01  2016/06/02-2016/07/01  2016/07/02-2016/08/01  2016/08/02-2016/09/01  2016/09/02-2016/10/01  2016/10/02-2016/11/01  2016/11/02-2016/12/01  2016/12/02-2017/01/01  2017/01/02-2017/02/01  2017/02/02-2017/03/01  2017/03/02-2017/04/01  2017/04/02-2017/05/01  2017/05/02-2017/06/01  2017/06/02-2017/07/01 
==========++========================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================
 assets   ||                     0                      0                      0                      0                      0                      0                    $-4                      0                      0                      0                      0                    $-8                      0                      0                      0                      0                      0                      0                   $-16                      0                      0                      0                      0                   $-32 
 expenses ||                     0                      0                      0                      0                      0                      0                     $4                      0                      0                      0                      0                     $8                      0                      0                      0                      0                      0                      0                    $16                      0                      0                      0                      0                    $32 
----------++------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
          ||                     0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0 
```

Note how 2015 is absent entirely. This is new expression, but i think that general nature of bug is the same...
```
$ hledger balance -f year-test.journal -p 'every 4th Apr'                                                                                                    ~/devel/haskell/darcs-get/hledger/examples
Balance changes in 2016/04/04-2018/04/03:

          || 2016/04/04-2017/04/03  2017/04/04-2018/04/03 
==========++==============================================
 assets   ||                  $-24                   $-32 
 expenses ||                   $24                    $32 
----------++----------------------------------------------
          ||                     0                      0 
```

After the fix:

All of 2015 is in monthly report:

$ hledger balance -f year-test.journal -p 'every 2nd day of month' 
Balance changes in 2015/01/02-2017/07/01:

          || 2015/01/02-2015/02/01  2015/02/02-2015/03/01  2015/03/02-2015/04/01  2015/04/02-2015/05/01  2015/05/02-2015/06/01  2015/06/02-2015/07/01  2015/07/02-2015/08/01  2015/08/02-2015/09/01  2015/09/02-2015/10/01  2015/10/02-2015/11/01  2015/11/02-2015/12/01  2015/12/02-2016/01/01  2016/01/02-2016/02/01  2016/02/02-2016/03/01  2016/03/02-2016/04/01  2016/04/02-2016/05/01  2016/05/02-2016/06/01  2016/06/02-2016/07/01  2016/07/02-2016/08/01  2016/08/02-2016/09/01  2016/09/02-2016/10/01  2016/10/02-2016/11/01  2016/11/02-2016/12/01  2016/12/02-2017/01/01  2017/01/02-2017/02/01  2017/02/02-2017/03/01  2017/03/02-2017/04/01  2017/04/02-2017/05/01  2017/05/02-2017/06/01  2017/06/02-2017/07/01 
==========++==================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================
 assets   ||                   $-1                      0                      0                      0                      0                    $-2                      0                      0                      0                      0                      0                      0                    $-4                      0                      0                      0                      0                    $-8                      0                      0                      0                      0                      0                      0                   $-16                      0                      0                      0                      0                   $-32 
 expenses ||                    $1                      0                      0                      0                      0                     $2                      0                      0                      0                      0                      0                      0                     $4                      0                      0                      0                      0                     $8                      0                      0                      0                      0                      0                      0                    $16                      0                      0                      0                      0                    $32 
----------++------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
          ||                     0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0                      0