In Go versions 1 up to and including Go 1.6,
ResponseRecorder.HeaderMap was both the map that handlers got access
to, and was the map tests checked their results against. That did not
mimic the behavior of the real HTTP server (Issue #8857), so HeaderMap
was changed to be a snapshot at the first write in
https://golang.org/cl/20047. But that broke cases where the Handler
never did a write (#15560), so revert the behavior.

Instead, introduce the ResponseWriter.Result method, returning an
*http.Response. It subsumes ResponseWriter.Trailers which was added
for Go 1.7 in CL 20047. Result().Header now contains the correct
answer, and HeaderMap is unchanged in behavior from previous Go
releases, so we don't break people's tests. People wanting the correct
behavior can use ResponseWriter.Result.

Fixes #15560
Updates #8857

Change-Id: I7ea9b56a6b843103784553d67f67847b5315b3d2
Reviewed-on: https://go-review.googlesource.com/23257Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>