Support Request being passed to fetch()

fetch.js

@@ -244,7 +244,7 @@
     return head
   }
 
-  Request.prototype.fetch = function() {
+  Request.prototype._fetch = function() {
     var self = this
 
     return new Promise(function(resolve, reject) {
@@ -325,8 +325,14 @@
   self.Request = Request;
   self.Response = Response;
 
-  self.fetch = function (url, options) {
-    return new Request(url, options).fetch()
+  self.fetch = function (input, init) {
+    var request
+    if (Request.prototype.isPrototypeOf(input)) {
+      request = input
+    } else {
+      request = new Request(input, init)
+    }
+    return request._fetch()
   }
   self.fetch.polyfill = true
 })();

test/test.js

@@ -146,6 +146,22 @@ suite('Request', function() {
     })
   })
 
+  test('fetch request', function() {
+    var request = new Request('/request', {
+      headers: {
+        'Accept': 'application/json',
+        'X-Test': '42'
+      }
+    })
+
+    return fetch(request).then(function(response) {
+      return response.json()
+    }).then(function(json) {
+      assert.equal(json.headers['accept'], 'application/json')
+      assert.equal(json.headers['x-test'], '42')
+    })
+  })
+
   test('construct with url', function() {
     var request = new Request('https://fetch.spec.whatwg.org/')
     assert.equal(request.url, 'https://fetch.spec.whatwg.org/')