Add 'Server' and 'User-Agent' default header fields.

pkg/http/test/client_test.dart

Index: pkg/http/test/client_test.dart
diff --git a/pkg/http/test/client_test.dart b/pkg/http/test/client_test.dart
index aa7781f1d6ef1ce3f7903354439b8b6e4e43a532..f6b0434ab0b5d5617d1ffe2484b14456ef251245 100644
--- a/pkg/http/test/client_test.dart
+++ b/pkg/http/test/client_test.dart
@@ -21,6 +21,7 @@ void main() {
       var request = new http.StreamedRequest("POST", serverUrl);
       request.headers[HttpHeaders.CONTENT_TYPE] =
         'application/json; charset=utf-8';
+      request.headers[HttpHeaders.USER_AGENT] = 'Dart';
 
       expect(client.send(request).then((response) {
         expect(response.request, equals(request));
@@ -37,6 +38,7 @@ void main() {
         'headers': {
           'content-type': ['application/json; charset=utf-8'],
           'accept-encoding': ['gzip'],
+          'user-agent': ['Dart'],
           'transfer-encoding': ['chunked']
         },
         'body': '{"hello": "world"}'