[7.x] Adds "when" method to the Mailable class

[elbojoloco]

This could be useful for example when conditionally attaching files, setting different subjects or maybe even using different views.

Here is an example use case:

return $this->from('[email protected]')
            ->when($cv, function ($message) use ($cv) {
                $message->attach($cv->getRealPath(), [
                    'as' => $cv->getClientOriginalName(),
                    'mime' => $cv->getClientMimeType(),
                ]);
            })
            ->subject('Example subject')
            ->view('mail.example');

Method signature is the exact same as the when method on the Illuminate\Database\Concerns\BuildsQueries trait, maybe for the future it's an idea to extract it (and similar methods) to a more generally reusable trait? One with a slightly different signature can also be found in Illuminate\Support\Collection, I believe they could be used interchangeably.

[lucasmichot]

Shouldn' $default be mixed?

[ahmedash95]

something like callable|null or string|int helps you know more what types does this method accept. mixed doesn't make it clear