[6.x] Changed Contract to Class

 - I have changed `\Illuminate\Contracts\Mail\Mailable` to `\Illuminate\Mail\Mailable`, since contract does not contains `cc` / `to` / `bcc` / `locale` methods, and in case if you call method only with contract, then we will get fatall error, since method is not present in the contract

Author: TBlindaruk

src/Illuminate/Mail/PendingMail.php

@@ -170,14 +170,14 @@ public function later($delay, MailableContract $mailable)
     /**
      * Populate the mailable with the addresses.
      *
-     * @param  \Illuminate\Contracts\Mail\Mailable $mailable;
+     * @param  \Illuminate\Mail\Mailable $mailable
      * @return \Illuminate\Mail\Mailable
      */
-    protected function fill(MailableContract $mailable)
+    protected function fill(Mailable $mailable)
     {
         return tap($mailable->to($this->to)
             ->cc($this->cc)
-            ->bcc($this->bcc), function ($mailable) {
+            ->bcc($this->bcc), function (Mailable $mailable) {
                 if ($this->locale) {
                     $mailable->locale($this->locale);
                 }