title: Silent notification is not working when app in background in iOS 11.4.1
body: <p>I'm trying to develop a messenger app, client needs to send only silent notification when app is in foreground or in background.</p>
  
  <p>It works perfectly in IOS 10.2.1 Verizon iPhone.</p>
  
  <p>After update to 11.4.1 notification works only in foreground but not in background.</p>
  
  <p>Payload: </p>
  
  <pre><code> {
       "to" : "/topics/channel_18",
       "data" : {
        "action" : "NOTIFY",
        "message" : "{"text":"test" }"
       },
       "content_available" : true
      }
  </code></pre>
  
  <p>code: </p>
  
  <pre><code> func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable: Any],
                       fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -&gt; Void) {
  
           Messaging.messaging().appDidReceiveMessage(userInfo)
          let action = userInfo["action"] as! String        
  
          // Configure the notification's payload.
          let content = UNMutableNotificationContent()
          content.title = NSString.localizedUserNotificationString(forKey: "Hello!", arguments: nil)
          content.body = NSString.localizedUserNotificationString(forKey: "testing", arguments: nil)
          content.sound = .none
  
          // Deliver the notification in five seconds.
          let trigger = UNTimeIntervalNotificationTrigger(timeInterval: 1, repeats: false)
          let request = UNNotificationRequest(identifier: "FiveSecond", content: content, trigger: trigger) // Schedule the notification.
          let center = UNUserNotificationCenter.current()
          center.add(request) { (error : Error?) in
              if let theError = error {
  
              }
          }
  
  
          completionHandler(UIBackgroundFetchResult.newData)
      }
  </code></pre>
  
  <p>I have gone through the link <a href="https://stackoverflow.com/questions/44796613/silent-pushes-not-delivered-to-the-app-on-ios-11">Silent pushes not delivered to the app on iOS 11</a> but I could not find the solution.</p>