Combine Request+RequestHeaders and Response+ResponseHeaders.

This cleans up a lot of the unnecessary moving parts in
HttpEngine. That class is still stuck with some complexity
around request and response bodies, but it's getting better.

We're also stuck escalating the visibility of a lot of
members in Request and Response. I've got some ideas on
how to fix this; that'll have to wait for a follow up.
Merge pull request #381 from square/jwilson_1229_requestheaders

Combine Request+RequestHeaders and Response+ResponseHeaders.