title: Swift 4 Label attributes
body: <p>I'm moving from swift 3 to swift 4.  I have UILabels that I am giving very specific text properties to the label.  I'm getting an 'unexpectedly found nil while unwrapping optional value' error when strokeTextAttributes is being initialized.  I'm totally lost to be frank.  </p>
  
  <p>In swift 3 the of strokeTextAttributes was [String : Any] but swift 4 threw errors until I changed it to what it is below.  </p>
  
  <pre><code>let strokeTextAttributes = [
      NSAttributedStringKey.strokeColor.rawValue : UIColor.black,
      NSAttributedStringKey.foregroundColor : UIColor.white,
      NSAttributedStringKey.strokeWidth : -2.0,
      NSAttributedStringKey.font : UIFont.boldSystemFont(ofSize: 18)
      ] as! [NSAttributedStringKey : Any]
  
  
  chevronRightLabel.attributedText = NSMutableAttributedString(string: "0", attributes: strokeTextAttributes)
  </code></pre>