Per RFC 7540, section 5.1 discussing the "idle" state: "Receiving any
frame other than HEADERS or PRIORITY on a stream in this state MUST be
treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR."x

Updates golang/go#16046

Change-Id: Ie0696059e76a092e483aea5ee017d9729339d309
Reviewed-on: https://go-review.googlesource.com/31736
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>