title: iOS 10 UNNotification doesn't show alert
body: <p>The code is very easy:</p>
  
  <pre><code>func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -&gt; Bool {
      UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .sound, .badge], completionHandler: { (granted, error) in
          if !granted {
              print("Not allowed")
          }
      })
  
      let content = UNMutableNotificationContent()
      content.title = "Alert"
      content.sound = UNNotificationSound.default()
  
      let trigger = UNTimeIntervalNotificationTrigger(timeInterval: 20, repeats: false)
  
      let request = UNNotificationRequest(identifier: "test", content: content, trigger: trigger)
  
      UNUserNotificationCenter.current().add(request, withCompletionHandler: nil)
      return true
  }
  </code></pre>
  
  <p>It works well on iOS 11, like this:
  <a href="https://i.stack.imgur.com/ozHs9.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ozHs9.png" alt="enter image description here"></a></p>
  
  <p>but on iOS 10, the alert doesn't show.</p>
  
  <p>On both iOS 10 and iOS 11, the sound did appear.</p>
  
  <p>My Xcode version is 9.2(9C40b)</p>
  
  <p>Any help is appreciated.</p>