Make the default HTTP server configuration more secure

tests/standalone/io/http_server_response_test.dart

Index: tests/standalone/io/http_server_response_test.dart
diff --git a/tests/standalone/io/http_server_response_test.dart b/tests/standalone/io/http_server_response_test.dart
index baef9ba6a6ebd6c5c13fc5fea39d6579a3b01999..967a5080652ab7dbfdc46dd8dc510f8c31a6d483 100644
--- a/tests/standalone/io/http_server_response_test.dart
+++ b/tests/standalone/io/http_server_response_test.dart
@@ -16,6 +16,7 @@ void testServerRequest(void handler(server, request),
                        {int bytes,
                         bool closeClient}) {
   HttpServer.bind("127.0.0.1", 0).then((server) {
+    server.defaultResponseHeaders.clear();
     server.listen((request) {
       handler(server, request);
     });