title: UIButton not respect Aspect Fill contentMode after resizing animation
body: <p>I am use auto layout. Following is the initial state of the view. </p>
  
  <p>In the centre is a button contained in a view. The button has contentMode Aspect Fill, and the image is set as the background image of the button.</p>
  
  <p><img src="https://i.stack.imgur.com/yeand.jpg" alt="enter image description here"></p>
  
  <p>Then I use the following code to transform the view, which will enlarge the centre card to fill the screen, and move the image to the top of the view:</p>
  
  <pre><code>cardTrailingSpaceConstraint.constant = 0
  cardLeadingSpaceConstraint.constant = 0
  cardView.removeConstraint(cardAspectRatioConstraint)
  let cardHeightConstraint = NSLayoutConstraint(item: cardView, attribute: .Height, relatedBy: .Equal, toItem: view, attribute: .Height, multiplier: 1.0, constant: 0)
  view.addConstraint(cardHeightConstraint)
  
  dishImageButton.removeConstraint(dishButtonBottomSpaceConstraint)
  let dishButtonHeightConstraint = NSLayoutConstraint(item: dishImageButton, attribute: .Height, relatedBy: .Equal, toItem: cardView, attribute: .Height, multiplier: 0.2, constant: 0)
  cardView.addConstraint(dishButtonHeightConstraint)
  
  cardView.setNeedsUpdateConstraints()
  UIView.animateWithDuration(0.7, delay: 0, usingSpringWithDamping: 0.7, initialSpringVelocity: 0.7, options: nil, animations: { [unowned self] () -&gt; Void in
      self.cardHeader.alpha = 0
      self.cardView.layer.cornerRadius = 0
      self.cardView.layoutIfNeeded()
  
      }) { [unowned self] (finished) -&gt; Void in
  
          }
  </code></pre>
  
  <p>The result is:</p>
  
  <p><img src="https://i.stack.imgur.com/sGt1j.png" alt="enter image description here"></p>
  
  <p>However, it is not what I want. The button is not respecting the contentMode and the image then get stretched.</p>
  
  <p>Can anyone tell me how to maintain the Aspect Fill contentMode of the button? </p>