Use the vendored ValidTrailerHeader function from x/net/http/httpguts to
check Trailer headers according to RFC 7230. The previous implementation
only omitted illegal Trailer headers defined in RFC 2616.

This CL adds x/net/http/httpguts from CL 104042 (git rev a35a21de97)

Fixes #23908

Change-Id: Ib2329a384040494093c18e209db9b62aaf86e921
Reviewed-on: https://go-review.googlesource.com/104075Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>