This compares the behavior of server handlers and the net/http
Transport in both HTTP/1 and HTTP/2 mode and verifies they're the
same.

This also moves some client<->server tests into clientserver_test.go.
Many of them were in serve_test.go or transport_test.go but were
basically testing both.

h2_bundle.go is an update of the golang.org/x/net/http2 code
from https://golang.org/cl/17204 (x/net git rev c745c36eab10)

Fixes #13315
Fixes #13316
Fixes #13317
Fixes other stuff found in the process too
Updates #6891 (http2 support in general)

Change-Id: Id9c45fad44cdf70ac95d2b89e578d66e882d3cc2
Reviewed-on: https://go-review.googlesource.com/17205Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>