Leave trailing slash when going up path (#781)

Leave trailing slash when going up path (#781)

Previously:

- `/some/path` → `/some`
- `/some/path/` → `/some`

Now:

- `/some/path` → `/some/`
- `/some/path/` → `/some/`

The convention on most modern sites seem to be to make a trailing slash
optional. But especially on older servers, no trailing slash means "view
file", while a trailing slash means "view directory". Removing the
trailing slash in such cases most likely results in a 404, while
preserving it gives a useful result.