mod_proxy_fcgi: handle the HTTP 412 use case

This is a follow up of http://svn.apache.org/r1752347;
ap_meet_conditions could return a 412 status that if not
handled causes subsequent bogus reads and wrong messages
logged (like AH01070). After a chat on dev@ the feedback
was to couple HTTP_NOT_MODIFIED with HTTP_PRECONDITION_FAILED,
but any other feedback is welcome.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1759984 13f79535-47bb-0310-9956-ffa450edef68