title: UITableViewCell top shadow is covered by UITableView header view
body: <p>I have a UITableView with a custom HeaderView (added via storyboard). I want to put a drop shadow on the first cell in the table view. Unfortunately, the header view covers the shadow. </p>
  
  <p>I am making the shadow in cellForRowAtIndexPath with:</p>
  
  <pre><code>[cell setClipsToBounds:NO];
  [cell.layer setMasksToBounds:NO];
  [cell.layer setShadowOffset:CGSizeMake(0, 2)];
  [cell.layer setShadowColor:[[UIColor blackColor] CGColor]];
  [cell.layer setShadowRadius:4.0];
  [cell.layer setShadowOpacity:1.0];
  [cell.layer setZPosition:10.0];
  </code></pre>
  
  <p>The shadow appears if I set the hidden state of the header view to YES. If the header if visible, it covers my shadow. I need the shadow to display in front of the header view. </p>
  
  <p>I have tried:</p>
  
  <pre><code>[self.tableView sendSubviewToBack:self.headerView];
  </code></pre>
  
  <p>Which has no effect. </p>
  
  <p>What's the correct way to accomplish this? Thanks!</p>