Add 'joined' field to cast in DateTime

Author: tolbon

src/Facebook/GraphNodes/GraphNode.php

@@ -150,7 +150,8 @@ public function shouldCastAsDateTime($key)
             'backdated_time',
             'issued_at',
             'expires_at',
-            'publish_time'
+            'publish_time',
+            'joined'
         ], true);
     }