Update router_server.js

Optionally just return for a route method which handles the response definitively (i. e. by calling this.response.end() or piping to the response). See http://stackoverflow.com/q/17073431/220060 for a use case.
Clarified what can be returned in the route function
Fixed a typo in the description
Support for 0.6.5 with node.js pipe stream and returning result