Allow conditional RequestHeader directives.

PR: 27951
Basically submitted by: vincent gryzor.com (Vincent Deffontaines)
Reviewed by: Jeff Trawick, Brad Nicholes, Andr� Malo


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@103865 13f79535-47bb-0310-9956-ffa450edef68