See RFC 7230.

Thanks to Régis Leroy for the report.

Change-Id: Ic1779bc2180900430d4d7a4938cac04ed73c304c
Reviewed-on: https://go-review.googlesource.com/11810Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/14250Reviewed-by: Andrew Gerrand <adg@golang.org>