Fix conversion to WebServiceWorkerResponse headers.

LayoutTests/http/tests/serviceworker/resources/fetch-response-xhr-worker.js

+self.addEventListener('fetch', function(event) {
+    var url = event.request.url;
+    if (url.indexOf('dummy?test') == -1) {
+      return;
+    }
+    event.respondWith(new Promise(function(resolve) {
+      var headers = new Headers;
+      headers.append('foo', 'foo');
+      headers.append('foo', 'bar');
+      resolve(new Response('hello world', {'headers': headers}));
+    }));
+});