Fixed type cast for delay type job

Author: munir131

src/PubSubQueue.php

@@ -145,7 +145,7 @@ public function later($delay, $job, $data = '', $subscriber = null)
         return $this->pushRaw(
             $this->createPayload($job, $data),
             $subscriber,
-            ['available_at' => $this->availableAt($delay)]
+            ['available_at' => (string) $this->availableAt($delay)]
         );
     }