Fix use of timeout property for synchronous XMLHttpRequests

Convert XMLHttpRequests to be asynchronous to allow for the use of the
timeout property. The timeout property can't be used for synchronous
requests in a document environment (window/iframe) or an
`InvalidAccessError` is thrown.

src/Request.js

@@ -123,9 +123,9 @@ export function request (url, params, callback, context) {
 
   // ie10/11 require the request be opened before a timeout is applied
   if (requestLength <= 2000 && Support.cors) {
-    httpRequest.open('GET', url + '?' + paramString);
+    httpRequest.open('GET', url + '?' + paramString, true);
   } else if (requestLength > 2000 && Support.cors) {
-    httpRequest.open('POST', url);
+    httpRequest.open('POST', url, true);
     httpRequest.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
   }