title: NavigationBar back button change text after popViewController
body: <p>I have a <code>UINavigationController</code> connected to a <code>UITabBarCotnroller</code>.
  When i push a <code>ViewController</code> and in that class i write this code in their <code>viewWillAppear</code> method</p>
  
  <pre><code>    self.navigationController?.navigationBarHidden = false
      let yourBackImage = UIImage(named: "Back.png")
      self.navigationController?.navigationBar.backIndicatorImage = yourBackImage
      self.navigationController?.navigationBar.backIndicatorTransitionMaskImage = yourBackImage
      self.navigationController!.navigationBar.backItem?.title = "";
  </code></pre>
  
  <p>By this code i am just setting a back button image with empty title.
  But when i push another screen over the current pushed screen and then tap back it again shows the title with text "Back"
  I am writing the above code in every view controller's <code>viewWillAppear</code> method which will be push.</p>