title: Error when instantiating a UIFont in an text attributes dictionary
body: <p>I'm trying to set the font of the <code>UIBarButtonItem</code> like so:</p>
  
  <pre><code>let barButton = UIBarButtonItem.appearance()
  barButton.setTitleTextAttributes([NSFontAttributeName: UIFont(name: "AvenirNext", size: 15], forState: UIControlState.Normal)
  </code></pre>
  
  <p>But it throws a compiler error saying: </p>
  
  <blockquote>
    <p>Cannot invoke 'init' with an argument list type '($T7, forState: UIControlState)`</p>
  </blockquote>
  
  <p>and I have no idea what that means. I have also tried </p>
  
  <pre><code>barButton.titleTextAttributesForState(UIControlState.Normal) =[NSFontAttributeName...]` 
  </code></pre>
  
  <p>but it appears that it isn't assignable</p>
  
  <p>How can I resolve this?</p>