mod_proxy: Avoid double slash with "ProxyPassReverse / ..." cases

trunk patch: http://svn.apache.org/viewvc?view=revision&revision=1386576
             http://svn.apache.org/viewvc?view=revision&revision=1386578
2.4.x patch: trunk patch works
+1: jim, druggeri, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1393199 13f79535-47bb-0310-9956-ffa450edef68