Merge r680082 from trunk:

Change r->content_type when mod_headers sets the Content-Type header
Submitted by: issac
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@771433 13f79535-47bb-0310-9956-ffa450edef68