[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 83acdda6a69c5e3c86fbcc70a643d834e5569210..bff23b8d009b4a1e2462e300ee247131a074df43 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);
     }