Buildbot change_hook

Master.cfg
---

```
104 c['www'] = dict(port=8020,
105                 plugins=dict(waterfall_view={}, console_view={}),
106                 change_hook_dialects={
107                                       'base': True,
108                                       'somehook': {'option1':True,
109                                                    'option2':False}})

 56 c['schedulers'].append(schedulers.SingleBranchScheduler(
 57                             name="waiter",
 58                             builderNames=["runtests"],
 59                             change_filter=util.ChangeFilter(category='default')))



```

How I call it. 
--- 

`curl -X POST -d '{"author":"aalvz", "comments":"my comment", "category":"default"}' http://192.168.33.20:8020/change_hook/base`
`curl -H "Content-Type: application/json" -X POST -d '{"author":"aalvz", "comments":"mycomment", "category":"default"}' http://192.168.33.20:8020/change_hook/base`

ERROR I GET
Error processing changes.

from twistd.log

2016-07-07 20:32:04+0000 [HTTPChannel,22,192.168.33.1] Attempting to load module buildbot.www.hooks.base
2016-07-07 20:32:04+0000 [HTTPChannel,22,192.168.33.1] processing changes from web hook
	Traceback (most recent call last):
	Failure: exceptions.AttributeError: 'NoneType' object has no attribute 'decode'