Merge r1584430, r1584434 from trunk:

mod_expires: don't add Expires header to error responses (4xx/5xx),
             be they generated or forwarded. PR 55669.


Follow up to r1584430.

Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1592999 13f79535-47bb-0310-9956-ffa450edef68