title: arraybuffer javascript ajax request returns null as response
body: <p>I attempting to download an excel file and check it before sending it on to the user. When I set a breakpoint on <code>var response - oReq.response</code>, I see that oReq.response is null. When I check my network tab, I see that the response is not null rather it is the excel file as expected. Why doesn't the excel file come through to the callback?</p>
  
  <pre><code>var oReq = new XMLHttpRequest();
  oReq.open("GET", service, true);
  oReq.responseType = "arraybuffer";
  
  oReq.onload = function (oEvent) {
      var response = oReq.response;
      if (response === parseInt(response, 10)) {
          alert("That query had no results");
          $("#dialog").dialog("close");
      }
      else {
          var blob = new Blob([response], { type: "application/vnd.ms-excel" });
          var objectUrl = URL.createObjectURL(blob);
          window.location = objectUrl;
          //saveAs(blob, 'Ad_Hoc_Results.xls');
          $("#dialog").dialog("close");
      }
  };
  oReq.send();
  </code></pre>
  
  <p>Response Headers:</p>
  
  <pre><code>Access-Control-Allow-Headers:Content-Type
  Access-Control-Allow-Methods:GET,PUT,POST,DELETE,OPTIONS
  Access-Control-Allow-Origin:*
  Cache-Control:no-cache
  Content-Disposition:attachment; filename=AdHocReport.xls
  Content-Length:6656
  Content-Type:application/vnd.ms-excel
  Date:Mon, 20 Jul 2015 19:56:50 GMT
  Expires:-1
  Pragma:no-cache
  Server:Microsoft-IIS/8.0
  X-AspNet-Version:4.0.30319
  X-Powered-By:ASP.NET
  X-SourceFiles:=?UTF-8?B?QzpcRGV2XE5KVEFcTGFuZUNsb3N1cmVBcGlcYXBpXHJlcG9ydFxHZXRBZEhvY1F1ZXJ5UmVwb3J0?=
  </code></pre>