Removed compress from accepted encodings.
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
Merge pull request #1916 from schlamar/accept-encoding

Fix Accept-Encoding in default headers