Stop appending a backslash if some trailing slash is present.

  For cross platform isapi we aught to find out what to add from
  apr, slash or backslash, but can't think of the trivial answer 
  at this hour.

PR: 15993

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@416291 13f79535-47bb-0310-9956-ffa450edef68