Sends send-to-device events to a set of client devices.

        Args:
            event_type (str): The type of event to send.
            messages (dict): The messages to send. Format should be
                <user_id>: {<device_id>: <event_content>}.
                The device ID may also be '*', meaning all known devices for the user.
            txn_id (str): Optional. The transaction ID for this event, will be generated
                automatically otherwise.

def send_to_device(self, event_type, messages, txn_id=None):
        """Sends send-to-device events to a set of client devices.

        Args:
            event_type (str): The type of event to send.
            messages (dict): The messages to send. Format should be
                <user_id>: {<device_id>: <event_content>}.
                The device ID may also be '*', meaning all known devices for the user.
            txn_id (str): Optional. The transaction ID for this event, will be generated
                automatically otherwise.
        """
        txn_id = txn_id if txn_id else self._make_txn_id()
        return self._send(
            "PUT",
            "/sendToDevice/{}/{}".format(event_type, txn_id),
            content={"messages": messages}
        )