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

pkg/shelf/README.md

 ## Web Server Middleware for Dart
 
 ## Introduction
 
 **Shelf** makes it easy to create and compose **web servers** and **parts of web
 servers**. How?
 
 * Expose a small set of simple types.
 * Map server logic into a simple function: a single argument for the request,
 the response is the return value.
@@ skipping 11 matching lines @@
 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}"');
 }
 ```
 
 ## Inspiration
 
 * [Connect](http://www.senchalabs.org/connect/) for NodeJS.
     * Read [this great write-up](http://howtonode.org/connect-it) to understand
       the overall philosophy of all of these models.
 * [Rack](http://rack.github.io/) for Ruby.
 * [WSGI](http://legacy.python.org/dev/peps/pep-3333/) for Python.