Call messenger::save_queue automatically

The queue in the messenger class is a useful thing to MOD authors and people who just do general work in phpBB. In the case of Titania, we had an interesting issue where we had the first parameter when creating the class as default (true) which happens to be "Use Queue," however messenger::save_queue() was not called causing a bug in Titania itself. What I propose is a usability improvement in the messenger API, calling messenger::save_queue() automatically at the end of messenger::send() if the queue is enabled.