Merge r399388 from trunk:

* Add Content-Type to headers_out if not already set. This ensures that
  the Content-Type of the cached entity gets restored correctly.

PR: 39266

Submitted by: rpluem
Reviewed by: rpluem, trawick, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@410932 13f79535-47bb-0310-9956-ffa450edef68