Fix spec bug: Section 5.3.1 says that self-dependencies should be
treated as a stream error of type PROTOCOL_ERROR.

Updates golang/go#16046

Change-Id: I8b5dc8808943dc96aac0c543c7032fa989ab9e0b
Reviewed-on: https://go-review.googlesource.com/31858Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>