title: UIScrollView with Autolayout has different size in iOS 7 and iOS 8
body: <p>its simple. in Xcode 6 I add an scrollView to to the view which covers entire screen. add constraints like this:</p>
  
  <p><img src="https://i.stack.imgur.com/xinOd.png" alt="enter image description here"></p>
  
  <p>then add a contentView in the scrollView:</p>
  
  <p><img src="https://i.stack.imgur.com/uB49T.png" alt="enter image description here"></p>
  
  <p>then add a sample label:</p>
  
  <p><img src="https://i.stack.imgur.com/kjYfH.png" alt="enter image description here"></p>
  
  <p>the problem is here when I run it on device with iOS 8: the scrollview width is <strong>320</strong> 
  but on device with iOS 7 width is <strong>320+16 = 336</strong> </p>
  
  <p>what am I doing wrong here? why scroll won't remain 320? this cause my custom collectionView flowLayout doesn't work well in iOS 7.</p>