title: SearchBar disappears from headerview in iOS 7
body: <p>I have a <code>tableview</code> for showing a list of devices in my application. When viewWillAppear is called, I add the <code>self.searchDisplayController.searchBar as a subview to a headerView</code>. I then assign <code>self.tableView.tableHeaderView = headerView</code>. It looks like this:
  <img src="https://i.stack.imgur.com/Z3iup.png" alt="enter image description here"></p>
  
  <p>I scroll the tableview down so that headerview goes out of view and then go to some other view controller by tapping on a cell. When I come back to this tableView, scroll up to the headerView, the searchBar becomes invisible, however on tapping the invisible area the searchDisplayController gets activated and the cancel button doesn't work. This happens for iOS 7 only. Why is this happening?
  <img src="https://i.stack.imgur.com/iockA.png" alt="enter image description here">
  <br>
  <strong>Note: It happens only if the headerView is out of the view when I come back to the tableViewController.</strong></p>