mod_ext_filter: Set additional environment variables for use by

the external filter.

PR:    20944

The missing code was identified by Andrew Ho, coded/tested by Jeff
Trawick, and verified yet again by Neil Fraser.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@100595 13f79535-47bb-0310-9956-ffa450edef68