MM_COERCE_KEYS_TO_STRINGS+sort_keys = same date keys are output

```python
import datetime
import rapidjson
rapidjson.dumps({"a":1, 3:4.5, datetime.date(2025,10,10):2, datetime.date(2025,11,11):3},
               mapping_mode=rapidjson.MM_COERCE_KEYS_TO_STRINGS, sort_keys=True)
==> '{"2025-11-11":2,"2025-11-11":3,"3":4.5,"a":1}'
```
note that both date keys are printed identically.