Fix issue #533

fetch.js

@@ -432,15 +432,21 @@
         }
         options.url = 'responseURL' in xhr ? xhr.responseURL : options.headers.get('X-Request-URL')
         var body = 'response' in xhr ? xhr.response : xhr.responseText
-        resolve(new Response(body, options))
+        setTimeout(function() {
+          resolve(new Response(body, options));
+        }, 0);
       }
 
       xhr.onerror = function() {
-        reject(new TypeError('Network request failed'))
+        setTimeout(function() {
+          reject(new TypeError('Network request failed'));
+        }, 0);
       }
 
       xhr.ontimeout = function() {
-        reject(new TypeError('Network request failed'))
+        setTimeout(function() {
+          reject(new TypeError('Network request failed'));
+        }, 0);
       }
 
       xhr.open(request.method, request.url, true)