title: Spring SseEmitter causes Cannot forward after response has been committed exception
body: <p>Given a simple controller with a method like this:</p>
  
  <pre><code>@RequestMapping(method = RequestMethod.GET, value = "{id}/update")
  public ResponseEntity&lt;SseEmitter&gt;  update() throws IOException {
      final SseEmitter  sseEmitter = new SseEmitter();
  
      return ResponseEntity.ok(sseEmitter);
  }
  </code></pre>
  
  <p>I tried it this way also:</p>
  
  <pre><code>@RequestMapping(method = RequestMethod.GET, value = "{id}/update")
  public SseEmitter  update() throws IOException {
      final SseEmitter  sseEmitter = new SseEmitter();
  
      return sseEmitter;
  }
  </code></pre>
  
  <p>But both way Tomcat 8.0.21 throws the aforementioned exception 30 seconds after the request issued. What is happening under the hood?</p>