Centralize the error-header collapsing done when sending out cached replies....

Centralize the error-header collapsing done when sending out cached replies. Mark the change in CHANGES as it may have unforseen fall out and/or may need to be reverted or done more subtle.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@649460 13f79535-47bb-0310-9956-ffa450edef68