title: How to cast AnyObject? to String in Swift 1.2
body: <p>Everything good until, I've downloaded the new version of Xcode 6.3 and Swift 1.2.</p>
  
  <p>But when I compile my app again, a lots of error came out.  I've dealt with most  of them but there is a specific error that I couldn't figure it out the solutions </p>
  
  <p>The error is "AnyObject? is not convertible to String?"</p>
  
  <p>Before updating Xcode this was absolutely fine :</p>
  
  <pre><code>let firstName = self.currentUser["firstName"] as? String
  </code></pre>