[ServiceWorker] Make Request class better conformance with the spec.

LayoutTests/http/tests/serviceworker/request-end-to-end.html

Index: LayoutTests/http/tests/serviceworker/request-end-to-end.html
diff --git a/LayoutTests/http/tests/serviceworker/request-end-to-end.html b/LayoutTests/http/tests/serviceworker/request-end-to-end.html
index 4e5f4656317f52852c605b8cbc377d7d78acee4a..fc0ea7397d1cc67ad415bba36de4efceda38433b 100644
--- a/LayoutTests/http/tests/serviceworker/request-end-to-end.html
+++ b/LayoutTests/http/tests/serviceworker/request-end-to-end.html
@@ -40,7 +40,7 @@ t.step(function() {
             'request.url should be passed to onfetch event.');
         assert_equals(event.data.method, 'GET',
                       'request.method should be passed to onfetch event.');
-        assert_equals(event.data.headers['User-Agent'], navigator.userAgent,
+        assert_equals(event.data.headers['user-agent'], navigator.userAgent,
                       'User-Agent header should be passed to onfetch event.')
         service_worker_unregister_and_done(t, scope);
     }