Fixes #11206 (that we accept invalid bytes)
Fixes #13624 (that we don't require a Host header in HTTP/1.1 per spec)

Change-Id: I4138281d513998789163237e83bb893aeda43336
Reviewed-on: https://go-review.googlesource.com/17892Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>