tests: Adjust FakeNetworkReply to work with all headers.
utils.http: Fix joining.
tests.utils.http: Add __init__.py
tests: Add tests for tests.utils.http
Change image/jpg Content-Type to image/jpeg.

This makes QtWebKit display images with a (nonstandard) image/jpg content-type,
e.g. http://m.c.lnkd.licdn.com/mpr/mpr/p/2/005/07b/08a/018245a.jpg (instead of
downloading them).