mod_cache does no longer send Content-Type header

for fresh content.

Followup to trunk r1361153+r1373447 resp. 2.4.x
r1373115+r1393191 (PR 53539, wrong Content-Type
for stale content).

Backport of r1439083 from trunk.

No CHANGES needed, because problem was introduced
after 2.4.3.

Submitted by: rjung
Backported by: rjung
Reviewed by: rjung, covener, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1439113 13f79535-47bb-0310-9956-ffa450edef68