Reverses the existing logic: a *seeker* URL will never end in a slash, and an index URL ALWAYS will.

Such that, given a sheer site with path:

/foo/bar.html
/foo/index.html

/foo/bar.html serves the file
/foo/bar.html/ 404 error
/foo/bar/        serves index.html
/foo/bar        redirects to /foo/bar/

This seems to me to be the correct way for it to work. @dpford can you test it out?