Merge r1546801 from trunk:

If the "value" argument is prefixed with expr=, parse it with ap_expr
rather than mod_headers' built-in format strings.


Submitted by: covener
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1588244 13f79535-47bb-0310-9956-ffa450edef68