GitLab

Adds tests for #122590 and updates x/net/http2 to git rev 6a8eb5e2b1 for:

     http2: call httptrace.ClientTrace.GetConn in Transport when needed
     https://golang.org/cl/122590

     http2: fire httptrace.ClientTrace.WroteHeaderField if set
     https://golang.org/cl/122816

     http2: compare Connection header value case-insensitively
     https://golang.org/cl/122588

This also includes the code for graceful shutdown, but it has no
public API surface via net/http, and should not affect any existing
code paths, as it's purely new stuff:

     http2: implement client initiated graceful shutdown
     https://golang.org/cl/30076

Fixes #19761
Fixes #23041

Change-Id: I5558a84591014554cad15ee3852a349ed717530f
Reviewed-on: https://go-review.googlesource.com/122591


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>