[5.4] Mailables that defined a delay property will honor it

[ipalaus]

This commit allows Mailables implementing ShouldQueue to define the $delay property in the Mailable class. This will allow to run Mail::queue(new ExampleMailable)); and honor the delay.

I know you can just use the Mail::later() that will respect the defined value (or assigned with $mailable->delay($delay)) but if we're already implementing the ShouldQueue contract and even the generated Mailable class with the command makes use of the Queueable I don't see a reason why you should be forced to use later().

This also allows to add a delay to any mailable class without having to specifically find where exactly we're sending the email.

[themsaid]

If we do this I think we'll have to allow defining delay in jobs and notifications as well, but imo delaying a queued task is a decision to be taken while dispatching since a single task could be delayed in part of the code base and dispatched instantly in another.

[ipalaus]

@themsaid this is already happening when you dispatch jobs afaik:

https://github.com/laravel/framework/blob/5.4/src/Illuminate/Bus/Dispatcher.php#L172-L174
https://github.com/laravel/framework/blob/5.4/src/Illuminate/Bus/Dispatcher.php#L180-L182

and I would expect a class implementing ShouldQueue to behave as the Job did, that's actually what I expected :)

This is kinda related to #18144 #18160 where Mail::send() would also respect the implementation of ShouldQueue.

[themsaid]

Yes but it wasn't meant to be used from inside the class, was never documented, it's set when you call delay() on the command.

I don't see a problem merging this PR, just wanted to share my opinion :). @taylorotwell will review it and see if he wants to merge