pkg/shelf - send Date header in shelf_io

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 80 matching lines @@
 may include body data for the 500 response, but this body data must not include
 information about the error that occurred. This ensures that unexpected errors
 don't result in exposing internal information in production by default; if the
 user wants to return detailed error descriptions, they should explicitly include
 middleware to do so.
 
 An adapter should include information about itself in the Server header of the
 response by default. If the handler returns a response with the Server header
 set, that must take precedence over the adapter's default header.
 
+An adapter should include the Date header with the time the handler returns a
+response. If the handler returns a response with the Date header set, that must
+take precedence.
+
 An adapter should ensure that asynchronous errors thrown by the handler don't
 cause the application to crash, even if they aren't reported by the future
 chain. Specifically, these errors shouldn't be passed to the root zone's error
 handler; however, if the adapter is run within another error zone, it should
 allow these errors to be passed to that zone. The following function can be used
 to capture only errors that would otherwise be top-leveled:
 
 ```dart
 /// Run [callback] and capture any errors that would otherwise be top-leveled.
 ///
 /// If [this] is called in a non-root error zone, it will just run [callback]
 /// and return the result. Otherwise, it will capture any errors using
 /// [runZoned] and pass them to [onError].
 catchTopLevelErrors(callback(), void onError(error, StackTrace stackTrace)) {
   if (Zone.current.inSameErrorZone(Zone.ROOT)) {
     return runZoned(callback, onError: onError);
   } else {
     return callback();
   }
 }
 ```
 
 ## 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.