Process a message before it's sent. If the payload isn't changed the given
@payload is just returned, otherwise g_bytes_unref() is called on the given
@payload and a new #GBytes is returned with the new data.
Extensions using reserved bits of the header will change them in @header.
Process a message before it's sent. If the payload isn't changed the given @payload is just returned, otherwise g_bytes_unref() is called on the given @payload and a new #GBytes is returned with the new data.
Extensions using reserved bits of the header will change them in @header.