title: If-Modied-Since header makes CORS OPTIONS request fail
body: <p>In the following code custom header (<code>If-Modified-Since</code>) makes ajax fail with status <code>HTTP/1.1 405 Not Allowed</code> of <code>OPTIONS</code> preflight request. However, if I remove the header, request works well as expected (I am not sure if <code>OPTIONS</code> is not sent at all or dev consoles just don't show it). Why is it so? (Same situations with googleapis hosted jquery).</p>
  
  <pre><code>http = new XMLHttpRequest();
  http.open('GET', 'http://code.jquery.com/jquery-2.1.4.min.js');
  http.setRequestHeader('If-Modified-Since', 'Sat, 29 Oct 1994 19:43:31 GMT')
  http.send();
  </code></pre>
  
  <p>Also, is there a way to send this header in xhr to jquery cdn to be able to get <code>304</code> reponse status?</p>