Previously, we panic'd on sending WINDOW_UPDATE on the half-closed-local
state. However, the RFC allows sending WINDOW_UPDATE in this state, so
we should no panic.

Change-Id: I702b2d5ad525837d7b8c650b7a2ed3f16371be63
Reviewed-on: https://go-review.googlesource.com/34498
Run-TryBot: Tom Bergan <tombergan@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>