make OutgoingMessage return static instead of self (botman#1197)

Author: ivankaizer

src/Messages/Outgoing/OutgoingMessage.php

@@ -30,7 +30,7 @@ public function __construct($message = null, Attachment $attachment = null)
      */
     public static function create($message = null, Attachment $attachment = null)
     {
-        return new self($message, $attachment);
+        return new static($message, $attachment);
     }
 
     /**