title: amp-list not printing returned data from the connected JSON file
body: <p>I'm trying to implement amp-list to allow a different currency depending on where the user is from. I've implemented the amp-list element and created a JSON file (which is a CORS url), containing the data using the correct syntax.</p>
  
  <p>The amp-list however is not printing the data, and instead printing a blank space. The HTML template is:</p>
  
  <pre><code>&lt;amp-list width="auto"
    height="50"
    layout="fixed-height"
    src="/amp-price/57938"&gt;
    &lt;template type="amp-mustache"&gt;
      &lt;p class="price align-center {{test}}"&gt;{{price}}&lt;/p&gt;
    &lt;/template&gt;
  &lt;/amp-list&gt;
  </code></pre>
  
  <p>And the JSON response is:</p>
  
  <pre><code>{"items": [{"price": "\u00a321.59", "test": "test"}]}
  </code></pre>
  
  <p>But the rendered HTML is:</p>
  
  <pre><code>&lt;amp-list width="auto" height="50" layout="fixed-height" src="/amp-price/57938" class="i-amphtml-element i-amphtml-layout-fixed-height i-amphtml-layout-size-defined i-amphtml-layout" aria-live="polite" style="height: 50px;"&gt;
    &lt;template type="amp-mustache"&gt;
      &lt;p class="price"&gt; - &lt;/p&gt;
    &lt;/template&gt;
  &lt;div class="i-amphtml-fill-content i-amphtml-replaced-content" role="list"&gt;&lt;/div&gt;&lt;/amp-list&gt;
  </code></pre>
  
  <p>The JSON response has all the correct AMP headers, and I'm not getting any AMP errors in the console.</p>
  
  <p>I've also followed the src link in the page source and it goes to the correct URL.</p>
  
  <p>Is there something simple I'm missing?</p>