prefetch_related causes api to return outdated data

When issuing a PUT request on a prefetched field, the old data is returned.

A failing test is included. I'll try to fix this myself, but any tips are appreciated :smile: 

**Resource:**
```
    class Meta:
       always_return_data = True
       queryset = Forum.objects.prefetch_related('moderators', 'members')
```

**Pseudo-Code:**
```
    Forum.objects.create()
    request.put('/api/forum/1/', {'moderators': [{'username': 'JannKleen', }, ], })
```

This should return the new moderator, but instead returns an empty list (the old value of the m2m field).