Switch to useing httpclient

This allows more fine-grained control over requests.
Functional tests around proxy header handling
Add tests for HTTP/1.0 support
Add pending test around default User-Agent
Pending test for support for the Via header
Add tests for verb, path, query and body handling
Merge pull request #12 from alphagov/proxy_function_tests

Proxy function tests

[Delivers #58407322]