Add `Request.clone()` and `Response.clone()` methods

https://fetch.spec.whatwg.org/#dom-response-clone

fetch.js

@@ -236,6 +236,27 @@
     this._initBody(body)
   }
 
+  function cloneBody(body) {
+    if (body._bodyText) {
+      return body._bodyText
+    } else if (body._bodyBlob) {
+      return body._bodyBlob
+    } else if (body._bodyFormData) {
+      return body._bodyFormData
+    }
+    return null
+  }
+
+  Request.prototype.clone = function() {
+    return new Request(cloneBody(this), {
+      method: this.method,
+      mode: this.mode,
+      credentials: this.credentials,
+      headers: new Headers(this.headers),
+      url: this.url
+    })
+  }
+
   function decode(body) {
     var form = new FormData()
     body.trim().split('&').forEach(function(bytes) {
@@ -279,6 +300,15 @@
 
   Body.call(Response.prototype)
 
+  Response.prototype.clone = function() {
+    return new Response(cloneBody(this), {
+      status: this.status,
+      statusText: this.statusText,
+      headers: new Headers(this.headers),
+      url: this.url
+    })
+  }
+
   Response.error = function() {
     var response = new Response(null, {status: 0, statusText: ''})
     response.type = 'error'

test/test.js

@@ -385,6 +385,35 @@ suite('Response', function() {
     })
   })
 
+  test('clone text response', function() {
+    var res = new Response('{"foo":"bar"}', {
+      headers: {'content-type': 'application/json'}
+    })
+    var clone = res.clone()
+
+    assert.notEqual(clone.headers, res.headers, 'headers were cloned')
+    assert.equal(clone.headers.get('content-type'), 'application/json')
+
+    return Promise.all([clone.json(), res.json()]).then(function(jsons){
+      assert.deepEqual(jsons[0], jsons[1], 'json of cloned object is the same as original')
+    })
+  })
+
+  ;(Response.prototype.arrayBuffer ? test : test.skip)('clone blob response', function() {
+    return fetch('/binary').then(function(response) {
+      return Promise.all([response.clone().arrayBuffer(), response.arrayBuffer()]).then(function(bufs){
+        bufs.forEach(function(buf){
+          assert(buf instanceof ArrayBuffer, 'buf is an ArrayBuffer instance')
+          assert.equal(buf.byteLength, 256, 'buf.byteLength is correct')
+          var view = new Uint8Array(buf)
+          for (var i = 0; i < 256; i++) {
+            assert.equal(view[i], i)
+          }
+        })
+      })
+    })
+  })
+
   test('error creates error Response', function() {
     var r = Response.error()
     assert(r instanceof Response)