Added a note in the mod_headers docs about Content-Type and setifempty

Added a note in the mod_headers docs about Content-Type and setifempty

This note has been added as a follow up of a stack overflow post
(thanks to Michael Allan for the research):

http://stackoverflow.com/questions/29398123/apache-2-4-set-mime-type-of-file-without-extension

After a chat in #httpd-dev it seems that the issue boils down to how %{CONTENT_TYPE}
is evaluated in util_expr_eval.c (r->content_type) vs how setifempty is (only a check
of the response headers). This particular behavior might be a bug or feature,
but it is worth to alert our users.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1769899 13f79535-47bb-0310-9956-ffa450edef68