prefetch_related causes api to return outdated data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).
In retrospect, TastyPie is quite happy to monkey about with "private" variables in django's internals elsewhere.  I'm still trying to wrap my head around the test case to understand how it is testing the specific problem.