Merge r1661448 from trunk:

core: Add expression support to ErrorDocument. Switch from a fixed
sized 664 byte array per merge to a hash table.

Submitted by: minfrin
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1670320 13f79535-47bb-0310-9956-ffa450edef68