Strip context path correctly if deployed to the root context

VRaptor is currently not stripping the context path correctly if the app is deployed to the root context path.

For ordinary context paths like `/myapp` it works fine:

    /myapp/somepage     ->     /somepath

But for the root context path `/` it is not correct:

    /somepage     ->     somepath

This pull request fixes the problem. I hope I found all the relevant places. :)