Adds a send_message function to the Dispatcher's
        dictionary of functions indexed by connection.

        Args:
            connection (str): A locally unique identifier
                provided by the receiver of messages.
            send_message (fn): The method that should be called
                by the dispatcher to respond to messages which
                arrive via connection.

def add_send_message(self, connection, send_message):
        """Adds a send_message function to the Dispatcher's
        dictionary of functions indexed by connection.

        Args:
            connection (str): A locally unique identifier
                provided by the receiver of messages.
            send_message (fn): The method that should be called
                by the dispatcher to respond to messages which
                arrive via connection.
        """
        self._send_message[connection] = send_message
        LOGGER.debug("Added send_message function "
                     "for connection %s", connection)