title: When adding nil to tableHeaderView cant show it again
body: <p>I have added table Header in the TableView in Storyboard, I want to hide/show this header. </p>
  
  <p>I have a segmented control, and want the header to show in one of the segments.</p>
  
  <p>To hide the header in one segment I used: <code>tableView.tableHeaderView = nil</code>
   and this hides it.
  In other segments, to unhide the header I used: </p>
  
  <pre><code>if tableView.tableHeaderView == nil {
      tableView.tableHeaderView = self.tableViewHeader
  }
  </code></pre>
  
  <p>but the header does not shown again. How to solve this?</p>