allow ap_invoke_handler() to pass-through AP_FILTER_ERROR as if it were

a reserved status code (OK/DECLINED/SUSPENDED). Prevents ap_die() from
seeing a 500 error when the http header filter has already taken
care of the proper error response



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@721679 13f79535-47bb-0310-9956-ffa450edef68