Always send the Host header first

[Pokechu22]

This pull request fixes #468.

I remembered to:

• Update or add unit tests if needed
• Update or add documentation/comments if needed
• Made sure stray files or whitespace didn't get committed
• If significant changes, branch from develop and set to merge into develop
• Read the guidelines for contributing

I ran tests by doing pip install . -t thematrix/ --no-dependencies followed by cd thematrix/ && nosetests --with-coverage --cover-package=wpull --cover-branches, but that didn't seem to actually run request_test.py (in that I didn't see any relevant error output if I changed the test but not the rest of the code). I also did python3 -m unittest wpull.protocol.http.request_test which did fail before my change and now passes.

Unfortunately, it seems like cloudflare doesn't hate me, so I'm not able to tell if this actually fixes anything in practice.