Always construct a new Request instance in fetch()

Section 6.5 of the spec:

> Let `request` be the associated request of the result of invoking
> the initial value of Request as constructor with `input` and `init`
> as arguments.

fetch.js

@@ -372,13 +372,7 @@
 
   self.fetch = function(input, init) {
     return new Promise(function(resolve, reject) {
-      var request
-      if (Request.prototype.isPrototypeOf(input) && !init) {
-        request = input
-      } else {
-        request = new Request(input, init)
-      }
-
+      var request = new Request(input, init)
       var xhr = new XMLHttpRequest()
 
       function responseURL() {