Fix incorrectly served 304 responses when expired cache entity

is valid, but cache is unwritable and headers cannot be updated.

Submitted by: Colm MacCarthaigh <colm stdlib.net>
Reviewed by: Justin Erenkrantz


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@231488 13f79535-47bb-0310-9956-ffa450edef68