pkg/shelf: case-insensitive headers, cleaner Request ctor, a lot more tests

pkg/shelf/example/example_server.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.
 
 import 'package:shelf/shelf.dart' as shelf;
 import 'package:shelf/shelf_io.dart' as io;
 
 void main() {
   var handler = const shelf.Stack().addMiddleware(shelf.logRequests())
       .addHandler(_echoRequest);
 
   io.serve(handler, 'localhost', 8080).then((server) {
     print('Serving at http://${server.address.host}:${server.port}');
   });
 }
 
 shelf.Response _echoRequest(shelf.Request request) {
-  return new shelf.Response.ok('Request for "${request.pathInfo}"');
+  return new shelf.Response.ok('Request for "${request.url}"');
 }