title: Auto Layout in Xcode 6 - Pinning view to edge of parent view creates -8 constraint
body: <p>In Xcode 6 when I try to pin an image view to the edge of <code>UICollectionViewCell</code>, auto-layout creates constraints of -8.0. What changed? I've tried to find release notes or documentation updates but haven't seen anything.</p>
  
  <ol>
  <li>Notice the imageview is up against the edges of the collectionview cell</li>
  </ol>
  
  <p><img src="https://i.stack.imgur.com/ClGWo.png" alt="image"></p>
  
  <ol start="2">
  <li>Frame X and Y are both zero</li>
  </ol>
  
  <p><img src="https://i.stack.imgur.com/J8uAe.png" alt="frame"></p>
  
  <ol start="3">
  <li>Adding constraints for leading space and trailing space to container margin results in a settings of -8.0</li>
  </ol>
  
  <p><img src="https://i.stack.imgur.com/9jecT.png" alt="constraints"></p>
  
  <p>Why is this happening and what is the preferred approach for edge to edge content?</p>