title: emberjs custom serialized field persisting differently
body: <p>I have the following serializer:</p>
  
  <p>import DS from 'ember-data';</p>
  
  <pre><code>export default DS.RESTSerializer.extend({
    serialize: function(record, options) {
      var json = this._super(record, options);
      json.is_foo = true;
      return json;
    }
  });
  </code></pre>
  
  <p>The idea is that I pass 'is_foo' back to the server. However, when I look at the payload the key has changed from <code>is_foo</code> to <code>is foo</code>. I'm not sure why this is. I've tried a number of variations, but can't get it to work. Is this the expected result or is there a bug somewhere in ember data?</p>