title: Polymer iron-ajax detail.response is null
body: <p>I'm new to Polymer. It looks great. I started using <code>iron-ajax</code> component to load some json from file (mocking rest service). The load function is triggered by a Polymer <code>ready</code> event. Everything goes the way it should go but <code>detail.response</code> object is <code>null</code>. I got no error, by Chrome's network consolle I can see a status <code>200</code> <code>GET</code> request and expected json data as response. Here is my <a href="https://github.com/fpezzati/yomoka_client" rel="nofollow">sandbox</a>.</p>