in per-dir, the filename will be internally redirected, so / is OK too.
don't add / to / in the non-perdir
match AP_IS_SLASH macro
followup to 1918651
Merges r1918651, r1918652, r1918663 from trunk
Reviewed by: covener, ylavic, rpluem
GH: close#458
git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1918668 13f79535-47bb-0310-9956-ffa450edef68