title: Restore navigationBar background image after setting it to [UIImage new]
body: <p>I needed a completely transparent navigation bar for the mapView so I did this:</p>
  
  <pre><code>[self.navigationController.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
  [self.navigationController.navigationBar setShadowImage:[UIImage new]];
  </code></pre>
  
  <p>That returns the desired effect, as seen here:</p>
  
  <p><img src="https://i.stack.imgur.com/9Ri8S.png" alt="enter image description here"></p>
  
  <p>Now I have a problem when I go to any other because my navigationBar remains transparent:</p>
  
  <p><img src="https://i.stack.imgur.com/Vwbzm.png" alt="enter image description here"></p>
  
  <p>How do I restore default settings of the navigationBar's backgroundImage and shadowImage?</p>