Fix test that reads "Date" header on IE

IE caches xhr responses pretty aggressively and after restoring the
previous response from cache, it blanks out the "Date" header (probably
by design). The solution for this test is to use a cache-busting query
parameter.

test/test.js

@@ -31,7 +31,7 @@ asyncTest('sends request headers', 2, function() {
 })
 
 asyncTest('parses response headers', 2, function() {
-  fetch('/headers').then(function(response) {
+  fetch('/headers?' + new Date().getTime()).then(function(response) {
     equal(response.headers.get('Date'), 'Mon, 13 Oct 2014 21:02:27 GMT')
     equal(response.headers.get('Content-Type'), 'text/html; charset=utf-8')
     start()