Updates x/net/http2 to git rev 5916dcb1 for:

* http2, lex/httplex: make Transport reject bogus headers before sending
  https://golang.org/cl/23229

* http2: reject more trailer values
  https://golang.org/cl/23230

Fixes #14048
Fixes #14188

Change-Id: Iaa8beca6e005267a3e849a10013eb424a882f2bb
Reviewed-on: https://go-review.googlesource.com/23234Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>