Updates http2 to x/net/http2 git rev 1087133bc4a for:

  http2: reject DATA frame before HEADERS frame
  https://golang.org/cl/56770

  http2: respect peer's SETTINGS_MAX_HEADER_LIST_SIZE in ClientConn
  https://golang.org/cl/29243

  http2: reset client stream after processing response headers
  https://golang.org/cl/70510

Also updated TestRequestLimit_h2 as the behavior changed slightly due
to https://golang.org/cl/29243.

Fixes #13959
Fixes #20521
Fixes #21466

Change-Id: Iac659694f3a48b8bd485546a4f96a932e3056026
Reviewed-on: https://go-review.googlesource.com/71611
Run-TryBot: Tom Bergan <tombergan@google.com>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>