TypeError: {123: 456} is not JSON serializable

Stock JSON module can handle `int`-keyed `dict`s:
```python
import json
print(json.dumps({123:456}))
==> {"123": 456}
```
Rapidjson cannot:
```python
import rapidjson as json
print(json.dumps({123:456}))
==>
       import rapidjson as json
---->  print(json.dumps({123:456}))

TypeError: {123: 456} is not JSON serializable
```