Small bug fix in multipart generation

This PR should fix an encondig bug with grammars and dicts

```
/home/ckut/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/att-codekit-4.0.1/lib/att/codekit/service.rb:197:in `block in generateMultiPart': incompatible character encodings: UTF-8 and ASCII-8BIT (Encoding::CompatibilityError)
```