title: AVAssetDownloadURLSession memory leak
body: <p>I used AVAssetDownloadURLSession to do background download.
  Everything worked fine but I always get memory leak from delegate.</p>
  <pre><code>NSURLSessionConfiguration *configuraton = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:url];
  
  AVAssetDownloadURLSession *downloadSession = [AVAssetDownloadURLSession sessionWithConfiguration:configuraton
                                                                             assetDownloadDelegate:self
                                                                                     delegateQueue:NSOperationQueue.mainQueue];
  
  [NSUserDefaults.standardUserDefaults setObject:@&quot;some obj&quot;
                                          forKey:url];
  
  NSURL *downloadUrl = [NSURL URLWithString:url];
  
  AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:downloadUrl
                                              options:nil];
  AVAssetDownloadTask *downloadTask = [downloadSession assetDownloadTaskWithURLAsset:asset
                                                                          assetTitle:@&quot;SomeTitle&quot;
                                                                    assetArtworkData:nil
                                                                             options:nil];
  [downloadTask resume];
  </code></pre>
  <p>like</p>
  <blockquote>
  <p>[__NSCFBackgroundAVAssetDownloadTask _onqueue_didLoadTimeRange:totalTimeRangesLoaded:timeRangeExpectedToLoad:]</p>
  <p>[__NSCFBackgroundAVAssetDownloadTask _onqueue_didResolveMediaSelectionPropertyList:]</p>
  </blockquote>
  <p>and I can't find some information about my memory leaks.
  Any help?</p>