Incompatible IntEnum encoding

python-rapidjson behaves differently from json and ujson when encoding IntEnums.
```
import enum
import json
import ujson
import rapidjson

class SomeInt(enum.IntEnum):
    val = 123


print([json.dumps(SomeInt.val), ujson.dumps(SomeInt.val), rapidjson.dumps(SomeInt.val)])
```
This code produces:
`['123', '123', 'SomeInt.val']`