title: addToBackState() doesn't restore fragment state
body: <p>When I replace <code>fragmentA</code> by <code>fragmentB</code> I use this code:</p>
  
  <pre><code> fragmentManager
          .beginTransaction()
          .replace(R.id.content_frame, new fragmentB())
          .addToBackStack(null)
          .commit();
  </code></pre>
  
  <p>But when I press back button <code>fragmentA</code> doesn't keep its previous state but reloads from the beginning. 
  How can I restore previous <code>fragmentA</code> state?</p>