avoid adding multiple subrequest filters when there are nested subrequests.

Multiple copies of the filter were not stripped properly 
during ap_fast_internal_redirect() which left the EOS buckets
stripped out of the brigade. This results in the end-chunk never
going out on the wire for a chunked response.

observed with mainreq -> directoryindex -> FallbackResource

PR58292



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1698239 13f79535-47bb-0310-9956-ffa450edef68