Fix HTTP Response to use cStringIO, adding Unicode support.

Change http_response's create_file_descriptor to get unicode support, avoiding tests failing when reading from Cassette at:

env/local/lib/python2.7/site-packages/cassette/http_response.py:55: in create_file_descriptor
    fp = io.BytesIO(content)
E   TypeError: 'unicode' does not have the buffer interface