[8.x] Prevent $mailer being reset when testing mailables that implement ShouldQueue

[JayBizzle]

class Signup extends Mailable implements ShouldQueue
{
    // snip
}

Mail::mailer('mail-connection')
    ->to('[email protected]')
    ->send(new Signup());

public function send_signup_using_correct_mailer()
{
    Mail::fake();

    // snip

    Mail::assertQueued(Signup::class, function ($mail) {
        return $mail->mailer == 'mail-connection'; // FAILS: It is actually NULL
    });
}

When calling send(), the $mailer is set on the Mailable instance, then immediately reset to null on the MailFake (which it should do).

The problem occurs when the Mailable implements ShouldQueue. After the $mailer property has been reset, the queue() method then sets the $mailer again on the Mailable, but this time it is now NULL.

This change fixes that.