Improving HttpProxyResponse tests

Signed-off-by: Alexandre Barbosa <alexandreab@live.com>
Signed-off-by: Carlos Oliveira <carlospecter@gmail.com>
Tests for charset in Content-Type

Added response factory as a parameter to patcher so that it returns
proxy response instead of a magic mock
Tested secure and unsecure host replacement
Refactored tests and factory to work with Python 3

Had to refactor tests to work on both Python 3 and Django 1.6 besides
Python 2 and Django 1.7
Tests for response header, status code and content

Testing whether the response headers returned are not in the Hop by Hop
headers dict, if the status code return is the same as the sent one and
whether the content remained the same
Merge pull request #8 from carloshfoliveira/tests

Tests for unicode_content inside response