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

pkg/shelf/example/example_server.dart

Index: pkg/shelf/example/example_server.dart
diff --git a/pkg/shelf/example/example_server.dart b/pkg/shelf/example/example_server.dart
index 781fb0c177b00b69de7e2026ca024b968a0e441d..49fda9eb4eb82301d177ee38dc07c53dc218faad 100644
--- a/pkg/shelf/example/example_server.dart
+++ b/pkg/shelf/example/example_server.dart
@@ -15,5 +15,5 @@ void main() {
 }
 
 shelf.Response _echoRequest(shelf.Request request) {
-  return new shelf.Response.ok('Request for "${request.pathInfo}"');
+  return new shelf.Response.ok('Request for "${request.url}"');
 }