PUTing a non-existant object in multiple m2m resource causes an error

This is actually the test that was lined out in #1035 (cc: @gjcourt @SeanHayes )
The expected behaviour when sending a PUT request like the one below is that a new user resource gets created (like it does if you were to add the user to just one m2m resource):
```
        user_data = {
            'username': 'valid but unique',
            'email': 'valid.unique@exmaple.com',
            'password': 'junk',
            }
        forum_data = {'members': [user_data_1, ],
                      'moderators': [user_data_1, ]}

        request.path = reverse('api_dispatch_detail', kwargs={'pk': forum.pk,
                                                              'resource_name': resource._meta.resource_name,
                                                              'api_name': resource._meta.api_name})

        response = resource.put_detail(request)

```