Fix boolean validation when string is passed as value

the following example:

```python
class BoolSerializer(serializers.Serializer):
    bool_field = serializer.BooleanField()
```

does behave in such case:

```Python
>>> s = BoolSerializer(data={'bool_field': 'somestring'})
>>> s.is_valid()
>>> True
```
Incorrect behaviour if value is `True`, `False`, 1 and 0.