Add support for HttpServer.defaultResponseHeaders to HttpMultiServer.

pkg/http_multi_server/lib/http_multi_server.dart

Index: pkg/http_multi_server/lib/http_multi_server.dart
diff --git a/pkg/http_multi_server/lib/http_multi_server.dart b/pkg/http_multi_server/lib/http_multi_server.dart
index 76c3eba1a43305d71ab71efc1535eadb3de48c7e..7075ab4736f0a80fd8567ad693bc5c09d2b50305 100644
--- a/pkg/http_multi_server/lib/http_multi_server.dart
+++ b/pkg/http_multi_server/lib/http_multi_server.dart
@@ -7,6 +7,7 @@ library http_multi_server;
 import 'dart:async';
 import 'dart:io';
 
+import 'src/multi_headers.dart';
 import 'src/utils.dart';
 
 /// An implementation of `dart:io`'s [HttpServer] that wraps multiple servers
@@ -20,6 +21,11 @@ class HttpMultiServer extends StreamView<HttpRequest> implements HttpServer {
   /// The wrapped servers.
   final Set<HttpServer> _servers;
 
+  /// Returns the default value of the `Server` header for all responses
+  /// generated by each server.
+  ///
+  /// If the wrapped servers have different default values, it's not defined
+  /// which value is returned.
   String get serverHeader => _servers.first.serverHeader;
   set serverHeader(String value) {
     for (var server in _servers) {
@@ -27,8 +33,11 @@ class HttpMultiServer extends StreamView<HttpRequest> implements HttpServer {
     }
   }
 
-  HttpHeaders get defaultResponseHeaders =>
-      throw new UnsupportedError('defaultResponseHeaders not supported');
+  /// Returns the default set of headers added to all response objects.
+  ///
+  /// If the wrapped servers have different default headers, it's not defined
+  /// which header is returned for accessor methods.
+  final HttpHeaders defaultResponseHeaders;
 
   Duration get idleTimeout => _servers.first.idleTimeout;
   set idleTimeout(Duration value) {
@@ -61,6 +70,8 @@ class HttpMultiServer extends StreamView<HttpRequest> implements HttpServer {
   /// listened to when this is called.
   HttpMultiServer(Iterable<HttpServer> servers)
       : _servers = servers.toSet(),
+        defaultResponseHeaders = new MultiHeaders(
+            servers.map((server) => server.defaultResponseHeaders)),
         super(mergeStreams(servers));
 
   /// Creates an [HttpServer] listening on all available loopback addresses for