Convert any types (strings usualy) to integer

I have a serializer like this:

```python
class ProfileSerializer(serializers.HyperlinkedModelSerializer):
    ...
    id = serializers.IntegerField(source='user_id', read_only=True)
```

Sometimes user_id field has string type on put requests. JSON response gotten:
```javascript
{
    "id": "17", 
   ...
}
```
expected:
```javascript
{
    "id": 17, 
   ...
}
```