User information is missing when using rest_framework

The reason is simple:

https://github.com/getsentry/raven-python/blob/master/raven/contrib/django/client.py#L255

Here raven checks if the request object is of Type Request and rest_framework has its own request object that isn't a subclass of Djangos request object.

Now what do you prefer for a PR to explicitly check for restframework request, or have a duck typing check?
