Allow notification routing to depend on notification being sent

This adds support for the feature added in Laravel 5.6 for routing based on the notification being sent (laravel/framework#22289).

src/GcmChannel.php

@@ -38,7 +38,7 @@ public function __construct(Client $client, Dispatcher $events)
      */
     public function send($notifiable, Notification $notification)
     {
-        $tokens = (array) $notifiable->routeNotificationFor('gcm');
+        $tokens = (array) $notifiable->routeNotificationFor('gcm', $notification);
         if (empty($tokens)) {
             return;
         }