mod_proxy_balancer: Correct doubled-slash ProxyPassReverse

PR:           51489 51982
Backports:    r1386576, r1386578
Submitted by: jimjag
Reviewed by:  wrowe, druggeri, humbedooh



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1470592 13f79535-47bb-0310-9956-ffa450edef68