title: Why isn't the navigationItem.titleView working?
body: <p>I'd like to put in an UIButton in the center of the navigation bar. Based on the document, it appears I need to set title view, and make sure the leftBarButtonItem is nil. But it is not work for me. Here is the code.</p>
  
  <pre><code>UIButton *btn = [UIButton buttonWithType:UIButtonTypeRoundedRect];
  [btn setTitle:@"List" forState:UIControlStateNormal];
  self.navigationItem.leftBarButtonItem = nil;
  [self.navigationItem.titleView addSubview:btn];
  </code></pre>