Fix memory corruption problem with ap_custom_response() function.

Fix memory corruption problem with ap_custom_response() function.
The core per-dir config would later point to request pool data
that would be reused for different purposes on different requests.

This is based on an old 1.3 patch submitted by Will Lowe.
It needs a minor tweak before committing to 1.3, but he had
it pretty darn close.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@103120 13f79535-47bb-0310-9956-ffa450edef68