[5.4] Dispatch Illuminate\Mail\Events\MessageSent event

[CyrilMazur]

Related to proposal laravel/ideas#505. This PR dispatches a new Illuminate\Mail\Events\MessageSent event AFTER the message is actually sent (to the contrary of MessageSending which is dispatched BEFORE the message is sent).

Pros:

• MessageSent is dispatched only if the message is successfully sent. It is not dispatched if the message isn't successfully sent (ie the Mailer throws an exception).
• The $message variable contains the message id (SES) / transmission id (SparkPost) that's set into the swift message's headers after the message is sent.

I think the use case in the example from the official doc is a bit clumsy (https://laravel.com/docs/5.4/mail#events), because the listener for logging the message is executed whether the email is actually sent or not. There could be an error while actually sending the email (ex: network issue, temporary API error from Mandrill / SparkPost etc...) and MessageSending would still be triggered. MessageSent would be a better fit for this use case, because when it's triggered it's guaranteed the message was actually sent.

[tillkruss]

Would you mind adding a test?

[CyrilMazur]

Done. The test checks that both MessageSending and MessageSent events are fired. I also handled the case when $this->events is null.

[CyrilMazur]

Thank you!