title: How to apply different contentInset only for contents cells but not for a header in UICollectionView
body: <p>I use insetForSectionAtIndex method to set contentInset for a section in my collection view and I don't want to apply that inset to a header of the section.  I need the header width to be as wide as the screen.</p>
  
  <p>ConentInset</p>
  
  <pre><code>func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAtIndex section: Int) -&gt; UIEdgeInsets {
      if section == 1 {
          return UIEdgeInsets(top: 0, left: 15, bottom: 0, right: 15)
      }
  
      return UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
  }
  </code></pre>
  
  <p>Header</p>
  
  <pre><code>override func collectionView(collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, atIndexPath indexPath: NSIndexPath) -&gt; UICollectionReusableView {
  
      let header = collectionView.dequeueReusableSupplementaryViewOfKind(kind, withReuseIdentifier: opsMainDescriptionSegmentedControlCellId, forIndexPath: indexPath) as! MyHeader
  
      return header
  }
  </code></pre>