Ensure req.params is correctly populated for Routers with a non-default root path.

Found what I believe to be a bug after pulling down the changes in https://github.com/yui/yui3/pull/1198 where the route regex's aren't correctly matched (and hence the req.params aren't correctly populated) if the Router has a non-default root path. This was because the route regex is run over the full path being dispatched rather than the rootless path.