title: How to expand UICollectionView contentSize when paging enable?
body: <p>I'm making a simple <code>UICollectionView</code> with a paging mechanism enabled, and everything works fine. Though, when scroll to the last page the number of the cells are not fully visible in the screen, and the last page contains some cells of the previous page.</p>
  <p>How do I expand the <code>contentSize</code> of the <code>UICollectionView</code> so that the last page doesn't contain any cells of the previous page?</p>
  <p>An example <a href="https://github.com/lequysang/github_zip/blob/master/DragCollectionViewCell.zip" rel="nofollow noreferrer">here</a>: the <code>UICollectionView</code> scrolls horizontally with 6 cells, this way:</p>
  <ul>
  <li><p><strong>Page 1:</strong></p>
  <p><code>cell0 - cell1 - cell2 - cell3</code></p>
  </li>
  <li><p><strong>Page 2:</strong></p>
  <p><code>cell4 - cell5</code> is expected, but unexpectedly</p>
  <p><code>cell2 - cell3 - cell4 - cell5</code></p>
  </li>
  </ul>
  <p>How to change it?</p>
  <p><strong>SUMMARY:</strong></p>
  <p>I want to set</p>
  <p><code>collectionView.contentSize = numberOfPage * collectionView.frame</code></p>
  <p><strong>NOT</strong></p>
  <p><code>collectionView.contentSize = numberOfCell * (cellFrame + spacing)</code></p>