nitcorn: redirect requests of directories to an URI ending with '/'

Example: request sent to http://xymus.net/nit will be redirected to http://xymus.net/nit/ (xymus.net) has not been updated yet). It fixes the loading of other assets such as css files.
Tel que discuté, le root n'est pas visé par cete modification, seulement les sous dossiers.