Update Response __getstate__ implementation to be on par with django > 1.4

Recently I had to change how the response was pickled subclassing and overriding the __getstate__ method and noticed the following:

Django 1.4 onwards use the rendering_attrs property to exclude the list from being pickled, making it easy to subclass and override the property. Since the original Response from drf does not use the same logic I thought it would be good to bring it on par with the newer SimpleTemplateResponse implementation

it's my first contribution ever so I'm happy to change anything you need
It'd be good to have a comment explaining the need to for this and linking to the relevant part of the Django docs.