title: how to get the absolute position of a section header in tableview？
body: <p>how to get the absolute position of a section header in tableview？</p>
  
  <p>I have already used the <code>tableView:viewForHeaderInSection:</code> to get the header's view.But now i need to get the absolute position of this view in the screen.
  the return value's frame is not the absolute position in the screen.I also tried to call superview,but it seems not work.</p>