title: Isomorphic fetch gives CORS error
body: <p>I am trying to covert my jQuery ajax call to use isomorphic fetch instead </p>
  
  <p>My jQuery ajax call looks like this:</p>
  
  <pre><code>$.ajax({
      url: 'myURL',
      dataType: "script",
      cache: true,
      success() {
          notify();
      }
  });
  </code></pre>
  
  <p>and my isomorphic fetch looks like this: </p>
  
  <pre><code> var myHeaders = new Headers();
   myHeaders.append('Content-Type', 'application/javascript');
   fetch('myURL', {
      headers: myHeaders,
      mode: 'cors',
      cache: 'default'
   }).then(function(){
       notify();
   }).catch(function(error) {
       console.log('request failed', error)
   })
  </code></pre>
  
  <p>In the isomorphic-fetch version I am getting a CORS error in the browser: </p>
  
  <p>"Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin '<a href="https://localhost:8080" rel="nofollow noreferrer">https://localhost:8080</a>' is therefore not allowed access. The response had HTTP status code 501. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled."</p>
  
  <p>However, the ajax call works fine. </p>
  
  <p>What am I doing wrong?</p>