[Fetch] Response.clone should tee the body stream.

LayoutTests/http/tests/fetch/script-tests/request.js

 if (self.importScripts) {
   importScripts('../resources/fetch-test-helpers.js');
 }
 
 var URL = 'https://www.example.com/test.html';
 
 function size(headers) {
   var count = 0;
   for (var header of headers) {
     ++count;
@@ skipping 650 matching lines @@
           assert_equals(req.headers.get('Content-Type'),
                         'text/plain;charset=UTF-8');
         });
   }, 'MIME type for USVString');
 
 promise_test(function(t) {
     var req = new Request('http://localhost/',
                           {method: 'POST',
                            body: new Blob([''], {type: 'Text/Plain'}),
                            headers: [['Content-Type', 'Text/Html']]});
-    req = req.clone();
-    return req.blob()
-      .then(function(blob) {
-          assert_equals(blob.type, 'text/html');
+    var clone = req.clone();
+    return Promise.all([req.blob(), clone.blob()])
+      .then(function(blobs) {
+          assert_equals(blobs[0].type, 'text/html');
+          assert_equals(blobs[1].type, 'text/html');
           assert_equals(req.headers.get('Content-Type'), 'Text/Html');
+          assert_equals(clone.headers.get('Content-Type'), 'Text/Html');
         });
   }, 'Extract a MIME type with clone');
 
 promise_test(function(t) {
     var req = new Request('http://localhost/',
                           {method: 'POST',
                            body: new Blob([''], {type: 'Text/Plain'})});
     req.headers.set('Content-Type', 'Text/Html');
     return req.blob()
       .then(function(blob) {
           assert_equals(blob.type, 'text/plain');
           assert_equals(req.headers.get('Content-Type'), 'Text/Html');
         });
   },
   'MIME type unchanged if headers are modified after Request() constructor');
 
 promise_test(function(t) {
     var req = new Request('http://localhost/',
                           {method: 'POST',
                            body: new Blob([''], {type: 'Text/Plain'}),
                            headers: [['Content-Type', 'Text/Html']]});
     return req.blob()
       .then(function(blob) {
           assert_equals(blob.type, 'text/html');
           assert_equals(req.headers.get('Content-Type'), 'Text/Html');
         });
   }, 'Extract a MIME type (1)');
 
 done();