title: addObserver to AVPlayer never being called when AVPlayer Ready to play?
body: <p>I have added the observer to AVPlayer but it's not calling the function when avplayer status is ready to play. The function called twice when video start playing but both the time duration parameter of AVPlayer return 'nan' and I need to get duration when AVPlayer ready to play.</p>
  
  <pre><code>    playerController.player = player
      self.addChildViewController(playerController)
      self.view.addSubview(playerController.view)
      playerController.view.frame = self.view.frame
      playerController.showsPlaybackControls = false
      player.addObserver(self, forKeyPath: "status", options: NSKeyValueObservingOptions(), context: nil)
      player.play()
  
  override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer&lt;Void&gt;) {
      if player.currentItem?.status == AVPlayerItemStatus.ReadyToPlay {
        print("a")  
      }
  }
  </code></pre>