(guts) Fix nav blowout on pages like http://docs.puppetlabs.com/pe/latest/

Aw, I knew that indexOf test was too clever to trust. If a page pathname was an
exact prefix of other pages (like /pe/latest/ is a prefix of
/pe/latest/whatever.html), it would mark all those pages as the current page. So
now we'll use a reliably paranoid regex, which also handles the synonymity of /$
and /index.html$.