title: Swift leaks when using NSJSONSerialization
body: <p>I want to write an extension for <code>NSDictionary</code>, so I can easily create it from the data. I wrote it like this:</p>
  
  <pre><code>extension Dictionary {
      init?(data: NSData?) {
          guard let data = data else { return nil }
          // TODO: This leaks.
          if let json = (try? NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions())) as? Dictionary {
              self = json
          }
          else { return nil }
      }
  }
  </code></pre>
  
  <p>Can't find out why this leaks. Any ideas?</p>