MaxMessageSize too low for Cloud Pub/Sub

Cloud Pub/Sub allows responses of up to 20 MB, so the channel default of 4 MB causes deframing errors for users, as can be seen here https://code.google.com/p/cloud-pubsub/issues/detail?id=50 . MaxMessageSize should either be increased to at least 20 MB, or else users should be given the ability to change it themselves.