title: iOS "thread-id" doesn't group push notifications
body: <p>From the <a href="https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/TheNotificationPayload.html" rel="noreferrer">documentation</a>:</p>
  
  <blockquote>
    <p><code>thread-id</code> | <code>string</code> | When displaying notifications, the system visually groups notifications with the same thread identifier together.
    For remote notifications, the value of the threadIdentifier property is set to the value of this request header.</p>
  </blockquote>
  
  <p>Our push notification payloads:</p>
  
  <pre><code>{
      aps =     {
          alert =         {
              body = "Leeroy J asked you: Test Push Notification";
          };
          badge = 12;
          sound = default;
          "thread-id" = 9150;
      };
      n = "6kQ/0x6556";
      r = 9150;
  }
  
  {
      aps =     {
          alert =         {
              body = "Leeroy J re: Test Push Notification";
          };
          badge = 13;
          sound = default;
          "thread-id" = 9150;
      };
      n = "6l8/0x6582";
      p = 7197;
      r = 9150;
  }
  </code></pre>
  
  <p>Here's what it looks like in Message Center:</p>
  
  <p><a href="https://i.stack.imgur.com/QNtfm.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/QNtfm.jpg" alt="enter image description here"></a></p>
  
  <p>I expected them to be visually different than other notifications... either that or grouped together when other notifications come in between. In our testing, neither is the case.</p>
  
  <p>What am I doing wrong? Or am I misunderstanding this feature?</p>