title: fetch response.json() gives responseData = undefined
body: <p>When using fetch:<br></p>
  
  <pre><code>  fetch(REQUEST_URL, {
        method: 'get',
        dataType: 'json',
        headers: {
          'Accept': 'application/json',
          'Content-Type': 'application/json'
        }
      })
      .then((response) =&gt; 
        {
          response.json() // &lt;&lt; This is the problem
        })
      .then((responseData) =&gt; { // responseData = undefined
  
          console.log(responseData);
       });
       }).catch(function(err) {
          console.log(err);
        })
       .done();
  </code></pre>
  
  <p>The following works works, do you know why? :<br></p>
  
  <pre><code>    JSON.parse(response._bodyText)
  </code></pre>