Make HttpTestClient HTTP compatible

[Ptico]

HttpTestClient operates via HTTP. So it must send and receive HTTP Headers not CGI Meta-Variables

[Ptico]

Also, I bumped WebMock dependency, because it was too old and fail tests on modern environment

[Jesterovskiy]

@oestrich maybe you can help with review this PR?

[marnen]

This works nicely for request headers. Unfortunately, this expects "Content-Type" as a response header, so it doesn't work with other case variations even though Faraday (or some servers) returns lowercase headers.

[marnen]

...or maybe that's not the issue after all. Hmm. It's not parsing JSON responses, at any rate.