Updates http2 to x/net/http2 git rev c73622c77280

http2: always delay closing the connection after sending GOAWAY
https://golang.org/cl/71372

http2: Discard data reads on HEAD requests
https://golang.org/cl/72551

Fixes #18701
Fixes #22376

Change-Id: I2460cec64848992fff21790868b5fb8c91f050f2
Reviewed-on: https://go-review.googlesource.com/75210
Run-TryBot: Tom Bergan <tombergan@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>