title: amp-list throwing error 400
body: <p>Trying to implement amp-list, it works fine with the sample json file, but throwing me error 400 when I try to fetch data from a upstream api which is HTTPS and CORS enabled</p>
  
  <pre><code>&lt;amp-list width="auto" height="100" layout="fixed-height" src="URL"&gt; &lt;template type="amp-mustache" id="amp-template-id"&gt; &lt;div&gt;&lt;a href="{{hosted_url}}"&gt;{{title}}&lt;/a&gt;&lt;/div&gt; &lt;/template&gt; &lt;/amp-list&gt;
  </code></pre>