If the payload isn't changed the given payload is just returned, otherwise
methodGlib.Bytes.unref is called on the given payload and a new
glib.bytes.Bytes 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 methodGlib.Bytes.unref is called on the given payload and a new glib.bytes.Bytes is returned with the new data.
Extensions using reserved bits of the header will change them in header.