Null-bytes in dictionary keys

Follows up https://github.com/kenrobbins/python-rapidjson/issues/38

The issue persists for dictionary keys:

```
(Pdb) import json
(Pdb) json.dumps({'\x00': 'a'})
'{"\\u0000": "a"}'

(Pdb) import rapidjson
(Pdb) rapidjson.dumps({'\x00': 'a'})
'{"":"a"}'
```