def on_send(self, frame):
        """
        Add the heartbeat header to the frame when connecting, and bump
        next outbound heartbeat timestamp.

        :param Frame frame: the Frame object
        """
        if frame.cmd == CMD_CONNECT or frame.cmd == CMD_STOMP:
            if self.heartbeats != (0, 0):
                frame.headers[HDR_HEARTBEAT] = '%s,%s' % self.heartbeats
        if self.next_outbound_heartbeat is not None:
            self.next_outbound_heartbeat = monotonic() + self.send_sleep
Add the heartbeat header to the frame when connecting, and bump
        next outbound heartbeat timestamp.

        :param Frame frame: the Frame object