* mod_cache: Preserve the Content-Type in case of 304 response.

304 does not contain Content-Type and mod_mime regenerates
the Content-Type based on the r->filename. This later leads to original
Content-Type to be lost (overwriten by whatever mod_mime generates).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1665216 13f79535-47bb-0310-9956-ffa450edef68