title: Silent push notifications in iOS 11 ignored.
body: <p>if I try to send a push notification silent (no badge/sound/alert) and just content-available, as per specs for silent notifications, on iOS 11, the notification is ignored and not forwarded to the app.</p>
  
  <p>This is the notification</p>
  
  <pre><code>{
      "aps" : {
          "content-available" : 1,
      }, 
      "type" : "0"
  }
  </code></pre>
  
  <p>The device log has:</p>
  
  <pre><code>&lt;Error&gt;: Ignoring notification with no alert, sound or badge (it.my.app.dev): C146-3838
  </code></pre>
  
  <p>And more complete:</p>
  
  <pre><code>Dec 27 10:11:30 iPierX SpringBoard(UserNotificationsServer)[59] &lt;Notice&gt;: [it.my.app.dev] Received remote notification request C146-3838 [ hasAlertContent: 0, hasSound: 0 hasBadge: 0 hasContentAvailable: 1 hasMutableContent: 0 ]
  Dec 27 10:11:30 iPierX SpringBoard(UserNotificationsServer)[59] &lt;Notice&gt;: [it.my.app.dev] Deliver push notification request C146-3838
  Dec 27 10:11:30 iPierX SpringBoard(UserNotificationsServer)[59] &lt;Notice&gt;: [it.my.app.dev] Passing content-available push to Duet
  Dec 27 10:11:30 iPierX SpringBoard(DuetActivityScheduler)[59] &lt;Notice&gt;: SUBMITTING: &lt;private&gt;
  Dec 27 10:11:30 iPierX SpringBoard(UserNotificationsServer)[59] &lt;Error&gt;: Ignoring notification with no alert, sound or badge (it.my.app.dev): C146-3838
  Dec 27 10:11:30 iPierX SpringBoard(UserNotificationsServer)[59] &lt;Notice&gt;: [it.my.app.dev] Not saving push notification C146-3838 to store [ error=Error Domain=UNErrorDomain Code=1401 "Notification has no user-facing content" UserInfo={NSLocalizedDescription=Notification has no user-facing content} ]
  Dec 27 10:11:30 iPierX dasd(DuetActivitySchedulerDaemon)[1645] &lt;Notice&gt;: Submitted Activity: com.apple.pushLaunch.it.my.app.dev:EA2367 &lt;private&gt;
  Dec 27 10:11:30 iPierX dasd(DuetActivitySchedulerDaemon)[1645] &lt;Notice&gt;: Daemon Canceling Activities: {(
      com.apple.pushLaunch.it.my.app.dev:EA2367
  )}
  Dec 27 10:11:30 iPierX dasd(DuetActivitySchedulerDaemon)[1645] &lt;Notice&gt;: CANCELED: com.apple.pushLaunch.it.my.app.dev:EA2367 &lt;private&gt;
  </code></pre>