title: Popover doesn't center on button
body: <p>I am trying to center a popover on a button. I can't seem to figure out where I might be going wrong. Instead of the arrow being in the middle of the button, it is off center by half the width of the screen.</p>
  
  <pre><code> @IBAction func buttonClicked(sender: AnyObject){
      var popoverViewController = self.storyboard?.instantiateViewControllerWithIdentifier("ServiceOptions") as! ServiceOptionsPopover
      popoverViewController.delegate = self
      popoverViewController.modalPresentationStyle = .Popover
      popoverViewController.preferredContentSize   = CGSizeMake(300, 300)
  
      let popoverPresentationViewController = popoverViewController.popoverPresentationController
  
      popoverPresentationViewController?.permittedArrowDirections = .Up
      popoverPresentationViewController?.delegate = self
      popoverPresentationViewController?.sourceView = sender as! UIButton
      popoverPresentationViewController?.sourceRect = sender.frame
  
      presentViewController(popoverViewController, animated: true, completion: nil)
  }
  </code></pre>