net/http/httputil: ReverseProxy should pass on unannounced Trailers

net/http/httputil: ReverseProxy should pass on unannounced Trailers

Trailers that are not announced in the Trailer must be passed on to
the downstream client.

Rather than iterate over each and find missing trailer values,
this re-adds all trailers to the headers if there is a disparity
between the number of announced trailers and the final number.

This fixes #20437

Change-Id: I867e85f45feff68616a9a9bd6f65f12d73825eb7
Reviewed-on: https://go-review.googlesource.com/43712
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>