`dumps` with `WM_PRETTY` not indented by 4 spaces when keys are `int`

With `python-rapidjson==1.12`:

```python
import rapidjson

data = {1: "hi"}
rapidjson.dumps(str(data), write_mode=rapidjson.WM_PRETTY)
```

I expect it to print:

```
{
    1: "hi"
}
```

However, it prints:

```
"{1: \'hi\'}"
```

Is this a bug, or is this just expected behavior when keys are `int`?