Add support for HttpServer.defaultResponseHeaders to HttpMultiServer.

pkg/http_multi_server/test/http_multi_server_test.dart

Index: pkg/http_multi_server/test/http_multi_server_test.dart
diff --git a/pkg/http_multi_server/test/http_multi_server_test.dart b/pkg/http_multi_server/test/http_multi_server_test.dart
index a3d9062067bced9da28b8452eccec18c0212a906..0371c8973ef3d856c97d3bd4d58f4e6bf3df2c5c 100644
--- a/pkg/http_multi_server/test/http_multi_server_test.dart
+++ b/pkg/http_multi_server/test/http_multi_server_test.dart
@@ -60,6 +60,28 @@ void main() {
       }), completes);
     });
 
+    test("headers.set sets the value for all servers", () {
+      multiServer.defaultResponseHeaders.set(
+          "server", "http_multi_server test");
+
+      multiServer.listen((request) {
+        request.response.write("got request");
+        request.response.close();
+      });
+
+      expect(_get(subServer1).then((response) {
+        expect(response.headers['server'], equals("http_multi_server test"));
+      }), completes);
+
+      expect(_get(subServer2).then((response) {
+        expect(response.headers['server'], equals("http_multi_server test"));
+      }), completes);
+
+      expect(_get(subServer3).then((response) {
+        expect(response.headers['server'], equals("http_multi_server test"));
+      }), completes);
+    });
+
     test("connectionsInfo sums the values for all servers", () {
       var pendingRequests = 0;
       var awaitingResponseCompleter = new Completer();