title: indexPathsForSelectedItems not return IndexPath on Swift 3
body: <p>I am using Swift 3 to get <code>indexPath</code> for selected items. The code looks like this.</p>
  
  <pre><code>override func prepare(for segue: UIStoryboardSegue, sender: Any?)
  {
      let indexPaths = collectionView.indexPathsForSelectedItems!
      print(indexPaths) 
  }
  </code></pre>
  
  <p>But this one print out empty array such as <code>[]</code> rather than an array of selected items of collection view cell. How can I get the <code>indexPath</code> row of selected cell?</p>