This is required by RFC 7540 Section 8.1.
Bug pointed out by Mike Bishop.

Updates golang/go#19948

Change-Id: I58d4f499609fd493d33115f4e1f64caf4bcbbc03
Reviewed-on: https://go-review.googlesource.com/40630Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>