test.rb

When running this spec, I get an encoding compatibility error:

https://github.com/rubinius/rubinius/blob/master/spec/ruby/core/io/gets_spec.rb#L280

results in:

1)
IO#gets transcodes into the IO object's internal encoding, when set ERROR
Encoding::CompatibilityError: undefined conversion for '"\u00FE\u00FF\u0000l\u0000i\u0000n\u0000e"' from UTF-16 to UTF-8
  Rubinius::Type.compatible_encoding at kernel/common/type.rb:533
                        String#index at kernel/common/string.rb:2261
    IO::EachReader#read_to_separator at kernel/common/io.rb:1008
                 IO::EachReader#each at kernel/common/io.rb:965
                             IO#each at kernel/common/io.rb:1251
                             IO#gets at kernel/common/io.rb:1494
            { } in Object#__script__ at spec/ruby/core/io/gets_copy_spec.rb:30
   BasicObject(Object)#instance_eval at kernel/common/eval.rb:43
       { } in Enumerable(Array)#all? at kernel/common/enumerable.rb:329
                          Array#each at kernel/bootstrap/array.rb:76
              Enumerable(Array)#all? at kernel/common/enumerable.rb:329
               Integer(Fixnum)#times at kernel/common/integer.rb:196
                          Array#each at kernel/bootstrap/array.rb:76
                   Object#__script__ at spec/ruby/core/io/gets_copy_spec.rb:7
                         Kernel.load at kernel/common/kernel.rb:497
   BasicObject(Object)#instance_eval at kernel/common/eval.rb:43
                          Array#each at kernel/bootstrap/array.rb:76
    Rubinius::CodeLoader#load_script at kernel/delta/code_loader.rb:66
    Rubinius::CodeLoader.load_script at kernel/delta/code_loader.rb:152
             Rubinius::Loader#script at kernel/loader.rb:645
               Rubinius::Loader#main at kernel/loader.rb:799


I can repro with this script:

```
Encoding.default_internal = Encoding::US_ASCII
string = "\u00FE\u00FF\u0000l\u0000i\u0000n\u0000e".force_encoding(Encoding::UTF_16)

string.index("\n")
```

which returns:

An exception occurred running index_spec.rb:

undefined conversion for '"\xC3\xBE\xC3\xBF\x00l\x00i\x00n\x00e"' from UTF-16 to UTF-8 (Encoding::CompatibilityError)

Backtrace:

  Rubinius::Type.compatible_encoding at kernel/common/type.rb:533
                        String#index at kernel/common/string.rb:2261
                   Object#__script__ at index_spec.rb:4
    Rubinius::CodeLoader#load_script at kernel/delta/code_loader.rb:66
    Rubinius::CodeLoader.load_script at kernel/delta/code_loader.rb:152
             Rubinius::Loader#script at kernel/loader.rb:645
               Rubinius::Loader#main at kernel/loader.rb:799