Make partial_pipeline JSON serializable for django 1.6

With django 1.6 the default session serializer is changed from `django.contrib.sessions.serializers.PickleSerializer` to `django.contrib.sessions.serializers.JSONSerializer` this causes an exception when partial pipelines is used. 

The error that is thrown is:
```python
TypeError: MergeDict(<QueryDict: {u'password1': [u'test'], u'password2': [u'test'], u'email': [test@example.com']}>, <QueryDict: {}>) is not JSON serializable
```

This happens when the `response` key of the partial of saved to the session with a `MergeDict` as the value. I've added a conversion from `MergeDict` to `dict` in `DjangoStrategy`.