float precious issue

```
Python 3.5.4 (v3.5.4:3f56838, Aug  8 2017, 02:07:06) [MSC v.1900 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import rapidjson as rjson
>>> import json
>>> d = {'a': 0.1+0.2}
>>> d
{'a': 0.30000000000000004}
>>> json.dumps(d)
'{"a": 0.30000000000000004}'
>>> rjson.dumps(d)
'{"a":0.30000000000000007}'
```