alt-fetch plugin: bugfix, will set Response.url (at least in Node; ref. #7)

plugins/alt-fetch.js

@@ -108,7 +108,8 @@
                 var init = {
                     status:     response.status,
                     statusText: response.statusText,
-                    headers: {}
+                    headers: {},
+                    url: url
                 };
                 response.headers.forEach(function(val, header){
                     init.headers[header] = val;