Fix the return values on mod_cache_socache's store_headers() implementation

to return APR errors codes and not DECLINED. We are committed to using this
implementation by this point, and any error should force us to stand down.
Submitted by: trawick
Reviewed by: minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1391396 13f79535-47bb-0310-9956-ffa450edef68