title: SortableMixin doesn't sort when using a custom transform
body: <p>I decided to use UTC dates in my Ember app so by following the recommendation <a href="http://blog.skylight.io/bringing-sanity-to-javascript-utc-dates-with-moment-js-and-ember-data/" rel="nofollow">here</a> I created the following transform:</p>
  
  <p><strong>app/transforms/utc.js</strong></p>
  
  <pre><code>import DS from "ember-data";
  
  export default DS.Transform.extend({  
      serialize: function(value) {
          return value ? value.toJSON() : null;
      },
  
      deserialize: function(value) {
          return moment.utc(value);
      }
  });
  </code></pre>
  
  <p>I then used it in my model:</p>
  
  <p><strong>app/models/payment.js</strong></p>
  
  <pre><code>import DS from 'ember-data';
  
  export default DS.Model.extend({
      student: DS.belongsTo('student'),
  
      date: DS.attr('utc'),
      amount: DS.attr('number'),
  
      formattedDate: function() {
          var date = this.get('date');
          return moment(date).format("DD/MM/YYYY");
      }.property('date')
  });
  </code></pre>
  
  <p>Before using the <code>utc</code> transform I was able to sort the payments by using this function:</p>
  
  <pre><code>payments: (function() {
      return Ember.ArrayProxy.createWithMixins(Ember.SortableMixin, {
          sortProperties: ['date'],
          sortAscending: false,
          content: this.get('content.payments')
      });
  }).property('content.payments'),
  </code></pre>
  
  <p>Now, after using the <code>utc</code> transform, the above function doesn't work anymore. Any idea why?</p>