Trailers() returns the headers that were set by the handler after the
headers were written "to the wire" (in this case HeaderMap) and that
were also specified in a proper header called "Trailer".

Neither HeaderMap or trailerMap (used for Trailers()) are manipulated by
the handler code, instead a third stagingMap is given to the
handler. This avoid a reference kept by handler to affect the recorded
results.

If a handler just modify the header but doesn't call any Write or Flush
method from ResponseWriter (or Flusher) interface, HeaderMap will not be
updated. In this case, calling Flush in the recorder is enough to get
the HeaderMap filled.

Fixes #14531.
Fixes #8857.

Change-Id: I42842341ec3e95c7b87d7e6f178c65cd03d63cc3
Reviewed-on: https://go-review.googlesource.com/20047Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>