title: Problems on NSArray's -valueForKey: when its item is NSDictionary
body: <p>I have an array which contains items of <code>NSDictionary</code>, I want to transform the items to other objects, my first thought is <code>valueForKey:</code>, so I add a category method <code>toMyObject</code> for <code>NSDictionary</code>, and call for:</p>
  
  <p><code>[array valueForKey:@"toMyObject"]</code></p>
  
  <p>But it doesn't work as expect, it just returns the array of <code>NSNull</code>s.</p>
  
  <p>Any ideas to solve this problem if I don't want to enumerate the array?</p>