pkg/shelf: Fixed logic for setting Server header in `shelf_io`

pkg/shelf/lib/shelf_io.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// A Shelf adapter for handling [HttpRequest] objects from `dart:io`.
 ///
 /// One can provide an instance of [HttpServer] as the `requests` parameter in
 /// [serveRequests].
 ///
 /// The `dart:io` adapter supports request hijacking; see [Request.hijack].
@@ skipping 99 matching lines @@
 }
 
 Future _writeResponse(Response response, HttpResponse httpResponse) {
   httpResponse.statusCode = response.statusCode;
 
   response.headers.forEach((header, value) {
     if (value == null) return;
     httpResponse.headers.set(header, value);
   });
 
-  if (response.headers[HttpHeaders.SERVER] == null) {
-    var value = httpResponse.headers.value(HttpHeaders.SERVER);
-    httpResponse.headers.set(HttpHeaders.SERVER, '$value with Shelf');
+  if (!response.headers.containsKey(HttpHeaders.SERVER)) {
+    httpResponse.headers.set(HttpHeaders.SERVER, 'dart:io with Shelf');
   }
 
   if (!response.headers.containsKey(HttpHeaders.DATE)) {
     httpResponse.headers.date = new DateTime.now().toUtc();
   }
 
   return httpResponse.addStream(response.read())
       .then((_) => httpResponse.close());
 }
 
 // TODO(kevmoo) A developer mode is needed to include error info in response
 // TODO(kevmoo) Make error output plugable. stderr, logging, etc
 Response _logError(String message, [StackTrace stackTrace]) {
   var chain = new Chain.current();
   if (stackTrace != null) {
     chain = new Chain.forTrace(stackTrace);
   }
   chain = chain
       .foldFrames((frame) => frame.isCore || frame.package == 'shelf')
       .terse;
 
   stderr.writeln('ERROR - ${new DateTime.now()}');
   stderr.writeln(message);
   stderr.writeln(chain);
   return new Response.internalServerError();
 }