auth: Handle REQUEST_URI not being authoritative

If _SERVER{REQUEST_URI} does not start with a leading slash, add one. This
will ensure that the redirect URL offered after a successful login will
be to an absolute url rather than an implied relative one.

Fixes osTicket/osTicket-1.7#858
Merge pull request #861 from greezybacon/issue/858

auth: Handle REQUEST_URI not being authoritative

Reviewed-By: Peter Rotich <peter@osticket.com>