title: Having problems trying to call substringWithRange on a String optional
body: <p>I'm trying to call substringWithRange on an option String but after multiple experiments am still not able to get it to compile:</p>
  
  <pre><code>var mdn:String?
  var subscriber = CTSubscriber()
  var carrierToken = subscriber.carrierToken
  mdn = NSString(data:carrierToken, encoding:NSUTF8StringEncoding)
  let range:NSRange = NSRange(location: 0, length: 10)
  if mdn
  {
      let subString = mdn!.substringWithRange(range)
  }
  </code></pre>
  
  <p>This will result in the compilation error saying the value of the optional NSString is not unwrapped.
  I thought it already was unwrapped due to the !. 
  If I remove ! then I get an error saying String? does not have a member named substringWithRange.</p>