Ensure that .raw() is present after unpickling.

Some people will assume that .raw() is present, and they shouldn't get
AttributeErrors when they make that assumption on a pickled Response.
However, @kennethreitz has asked that we not be too dependent on
urllib3. For that reason, set to None.
Merge pull request #1949 from Lukasa/pickle

Pickled Responses should include a None value for the raw attribute