app: return MethodNotAllowed for non-POST requests
app: return MethodNotAllowed for non-POST requests

Change-Id: I6053506d2eeac54e46f2ebd8a82e79121ff1da9b
Reviewed-on: https://go-review.googlesource.com/27076
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/playground,golang/playground,golang/playground