title: Downcast from 'UIImage?' to 'UIImage' only unwraps optionals
body: <p>I'm creating a <code>UIButton</code> with an image,</p>
  
  <p>I have written the below code for that:</p>
  
  <pre><code>    let btnImg=UIButton.buttonWithType(UIButtonType.Custom) as UIButton
      let img  = UIImage(named: "turn_left") as UIImage
      btnImg.setTitle("Turn left", forState: UIControlState.Normal)
      btnImg.setImage(img, forState: UIControlState.Normal)
      btnImg.frame = CGRectMake(10, 150, 200, 45)
      self.view.addSubview(btnImg)
  </code></pre>
  
  <p>But I got the error below at <code>let img  = UIImage(named: "turn_left") as UIImage</code>:</p>
  
  <pre><code>Swift Compiler error:
   Downcast from 'UIImage?' to 'UIImage' only unwraps optionals; did you mean to use '!'?
  </code></pre>