Fix request's path_info when a rack app mounted at '/'.

When a rack app is mounted at '/', and we visit '/omg', the env['PATH_INFO'] will be 'omg' and be passed to the rack app. Then the rack app won't find the match path for 'omg'.

Tests with https://gist.github.com/Peeja/de5ce3ea12b264aaf792 has passed.

Fixes issue #15511.

cc @Peeja