mod_ssl/proxy: enable the SNI extension for backend TLS connections

Backports: r1175416
Submitted by: kbrand
Reviewed by: wrowe, rjung, trawick


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1497466 13f79535-47bb-0310-9956-ffa450edef68