[Fetch API] Implement combining of Headers with same keys.

third_party/WebKit/LayoutTests/http/tests/fetch/script-tests/headers.js

 if (self.importScripts) {
   importScripts('../resources/fetch-test-helpers.js');
 }
 
 test(function() {
     var expectedValueMap = {
       'content-language': 'ja',
       'content-type': 'text/html; charset=UTF-8',
       'x-fetch-test': 'response test field'
     };
@@ skipping 221 matching lines @@
         keys.push(key);
     assert_array_equals(keys, ['a', 'b', 'c'],
                         'The pairs to iterate over should be sorted.');
 }, 'Iteration order');
 
 test(function(t) {
     const headers = new Headers;
     headers.set('a', '1');
     headers.set('b', '2');
     headers.set('c', '3');
+    headers.append('a', '2');
+    headers.append('a', '3');
 
     const iterator = headers.entries();
 
     headers.delete('a');
     headers.set('d', '4');
 
     const keys = [];
-    for (let [key, value] of iterator)
+    const values = [];
+    for (let [key, value] of iterator){

[jsbell, 2016/12/12 22:38:31]

nit: space between ) and {

         keys.push(key);
+        values.push(value);
+      }

[jsbell, 2016/12/12 22:38:31]

nit: } should line up with for

     assert_array_equals(keys, ['a', 'b', 'c'],
                         'The pairs to iterate over should be the return ' +
                         'value of an algorithm that implicitly makes a copy.');
+    assert_array_equals(values, ['1,2,3', '2', '3'],
+                        "The values should be combined and separated by ','.");
 }, 'Iteration mutation');
 
 done();