mod_cache: follow up to r1591328.

Define the cache_merge_headers_out() function to merge r->err_headers_out into
r->headers_out and add the ones from r->content_type/encoding if available.
Use it in ap_cache_cacheable_headers_out() where the same is done and in
cache_save_filter() where this has to be done before updating the entry.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1594643 13f79535-47bb-0310-9956-ffa450edef68