title: NSURLSession cancel Task
body: <p>I create new NSURLSession with following configs</p>
  
  <pre><code> if (!self.session) {
              NSURLSessionConfiguration *config = [NSURLSessionConfiguration backgroundSessionConfiguration:[self uniquieIdentifier]];
              config.discretionary = NO;
              self.session = [NSURLSession sessionWithConfiguration:config delegate:self delegateQueue:[NSOperationQueue mainQueue]];
          }
  </code></pre>
  
  <p>and on after pressing a button I am trying to stop all current download tasks. </p>
  
  <pre><code>[[[self session] delegateQueue] setSuspended:YES];
  [[self session] invalidateAndCancel];
  </code></pre>
  
  <p>Nevertheless I get responses in delegate method didFinishDownloadingToURL, and I am pretty sure that no new sessions or download task are created after this point. How to stop all task from happening?</p>