title: [NSOperation cancelAllOperations]; does not stop the operation
body: <p>xCode 4.4.1 OSX 10.8.2, looks like [operation cancelAllOperations]; isn't working </p>
  
  <pre><code>- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
      NSOperationQueue *operation = [[NSOperationQueue alloc] init];
      [operation setMaxConcurrentOperationCount: 1];
      [operation addOperationWithBlock: ^{
          for (unsigned i=0; i &lt; 10000000; i++) {
              printf("%i\n",i);
             }
      }];
      sleep(1);
      if ([operation operationCount] &gt; 0) {
          [operation cancelAllOperations];
      }
  }
  </code></pre>
  
  <p>results 9999999</p>