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