[5.4] Fix regression on queueable mailables (#18160)

Refs: #18144

This fixes the infinite recursion the was happening. I have tested on
my local machine and looks good.

@tomcoonen Can you have a look at this?

src/Illuminate/Mail/Mailer.php

@@ -213,9 +213,9 @@ public function send($view, array $data = [], $callback = null)
      * Send the given mailable.
      *
      * @param  MailableContract  $mailable
-     * @return void
+     * @return mixed
      */
-    protected function sendMailable($mailable)
+    protected function sendMailable(MailableContract $mailable)
     {
         return $mailable instanceof ShouldQueue
                 ? $mailable->queue($this->queue) : $mailable->send($this);

src/Illuminate/Mail/SendQueuedMailable.php

@@ -49,7 +49,7 @@ public function __construct(MailableContract $mailable)
      */
     public function handle(MailerContract $mailer)
     {
-        $mailer->send($this->mailable);
+        $this->mailable->send($mailer);
     }
 
     /**