core: make the "default" parameter of the "ErrorDocument" option case insensitive.

PR 54419

trunk patch: http://svn.apache.org/r1465190
Submitted by: Tianyin Xu <tixu cs ucsd edu>
Reviewed by: jailletc36, fuankg, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1490594 13f79535-47bb-0310-9956-ffa450edef68