Fix JSON ClassLabel casting for integers
Loading a JSON dataset with ClassLabel feature types currently fails if the JSON data already has integers. Indeed currently it tries to convert the strings to integers without even checking if the data are not integers already.

For example this currently fails:
```python
from datasets import load_dataset, Features, ClassLabel

path = "data.json"
f = Features({"a": ClassLabel(names=["neg", "pos"])})
d = load_dataset("json", data_files=path, features=f)
```
data.json
```json
{"a": 0}
{"a": 1}
```

I fixed that by adding a line that checks the type of the JSON data before trying to convert them

cc @albertvillanova let me know if it sounds good to you