mod_disk_cache: When renegotiating an already cached Vary'd response, create

the refreshed .vary structure in the right directory.  Otherwise, we'll create
a new .vary layout underneath the already existing one - oops!

* modules/cache/mod_disk_cache.c
  (store_headers): Temporarily clear dobj->prefix if it is set to ensure that
  we create the .vary structure underneath the cache root.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@538992 13f79535-47bb-0310-9956-ffa450edef68