Partial update (PATCH) should return new values after successful save

When I try to use use partial update, the values are saved correctly but [response](https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/mixins.py#L69) has old values.

I think the problem is somewhere in [UpdateModelMixin](https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/mixins.py#L66).
