title: Backbone/Underscore sortBy is not sorting collection
body: <p>I have a list of users (six to be exact) in a collection with 'firstname', 'lastname' properties. Doing a fetch, the comparator below sorts them by 'firstname', and it works fine.</p>
  
  <pre><code>comparator : function (user) {
    return user.get("firstname").toLowerCase();
  }
  </code></pre>
  
  <p>But if I try to sort the collection later, by a different value i.e. 'lastname', it doesn't work. The order stays the same.</p>
  
  <pre><code>this.collection.sortBy(function(user) {
    return user.get("lastname").toLowerCase();
  });
  </code></pre>
  
  <p>What am i doing wrong? </p>
  
  <p><br>
  <strong>Update</strong></p>
  
  <hr>
  
  <p>So the data returned from sortBy IS sorted but that doesn't help me really as my view is linked to the collection. If i reset the collection and add the sorted array back to the collection it's comparator does it's job and sorts it back into 'firstname' order.</p>
  
  <pre><code>var sorted = this.collection.sortBy(function(user) {
    return user.get("lastname").toLowerCase();
  });
  </code></pre>