title: How to change nowPlayingInfo of MPNowPlayingInfoCenter retaining old artwork
body: <p>I noticed that setting new <code>nowPlayingInfo</code> of <code>[MPNowPlayingInfoCenter defaultCenter]</code> replaces <code>MPMediaItemPropertyArtwork</code> value every new perform <code>setNowPlayingInfo:</code>. But in standart iphone music.app it doesn't happend! So how to set <code>nowPlayingInfo</code> retaining old artwork?</p>
  
  <pre><code>NSMutableDictionary *mediaInfo = [NSMutableDictionary dictionaryWithObjectsAndKeys:recording.releaseGroup.title, MPMediaItemPropertyAlbumTitle, recording.releaseGroup.artist.title, MPMediaItemPropertyArtist, recording.title, MPMediaItemPropertyTitle, recording.lyrics, MPMediaItemPropertyLyrics, itemsCount, MPMediaItemPropertyAlbumTrackCount, itemNumber, MPMediaItemPropertyAlbumTrackNumber, nil];
  
  
   self.artwork = [[[MPMediaItemArtwork alloc] initWithImage:_coverView.image] autorelease];
  
  
  [mediaInfo setValue:_artwork forKey:MPMediaItemPropertyArtwork];
  
  
  [[MPNowPlayingInfoCenter defaultCenter] setNowPlayingInfo:mediaInfo];
  </code></pre>
  
  <p>This block is executed every change of track is playing</p>