title: cancelAllOperations doesn't work for [NSOperationQueue mainQueue]
body: <p><code>cancelAllOperations()</code> doesn't work on the mainQueue (the <code>cancel()</code> method is not called on the <code>NSOperation</code> object). Am I missing something?
  I have to iterate through all operations and call the <code>cancel()</code> method to get it work.</p>