title: Add a gesture recognizer to tableView header doesn't work?
body: <p>I know that a "table view header"(the most top-part of a table view) is a View
  So I try to add a UITapGestureRecognizer to it ,but it doesn't work...</p>
  
  <p>code is simple : </p>
  
  <pre><code>- (void)tap:(UITapGestureRecognizer *)recognizer
  {
      // do something
  }
  
  UITapGestureRecognizer *recognizer = [[UITapGestureRecognizer alloc]    initWithTarget:self action:@selector(tap:)];
  [self.tableView.tableHeaderView addGestureRecognizer:recognizer];
  </code></pre>
  
  <p>Any tips here to care ? thanks a lot </p>