title: Navigation Back Button does not show,how to add back button to UINavigationItem
body: <p>I add UINavigationBar via Library to view. I also add UINavigationItem to this NavigationBar.</p>
  
  <p>In viewController.m,I added the following code but back button doesn't show.</p>
  
  <pre><code>self.navigationItem.title = @"List";
  self.navigationItem.backBarButtonItem.title = @"Back";
  </code></pre>