Make RawHeaders, RequestHeaders and ResponseHeaders immutable.

This introduces a new, poorly-named class ResponseStrategy
that pulls some code out of ResponseHeaders. That was necessary
because the old method mutated itself and its parameters in
place.

Obvious follow-up for this is to combine ResponseHeaders with
Response, and RequestHeaders with Response.
Merge pull request #380 from square/jwilson_1229_immutable_headers

Make RawHeaders, RequestHeaders and ResponseHeaders immutable.