title: Clicking back button after a Fragment transaction using addToBackStack does nothing
body: <p>I want to be able to reverse a <code>replace</code> <code>FragmentTransaction</code> by using <code>addToBackStack()</code>:</p>
  
  <pre><code>FragmentTransaction fragmentTransaction = getActivity().getSupportFragmentManager().beginTransaction();         
  Fragment scheduleFragment = new ScheduleFragment();     
  fragmentTransaction.replace(R.id.content_container, scheduleFragment, "scheduleFragment");
  fragmentTransaction.addToBackStack("scheduleFragment");
  fragmentTransaction.commit();
  </code></pre>
  
  <p>but after that, clicking the back button does nothing.</p>
  
  <p>From the doc and it's supposed to reverse the transaction.</p>
  
  <p>What am I missing?</p>