Sub-requests are created and used with two purposes; sometimes

Sub-requests are created and used with two purposes; sometimes
simply to 'see' what a request would do; as to fill out an SSI,
validate access or similar - and is then discarded. And sometimes
as the precursor to becoming the actual request; e.g. when mod_dir
checks if an /index.html can be served for a '/'.

In the latter case it is important to preserve the output filters
'for real'; whereas in the first case they have to be reset to
purely the minimal proto filters (if at all). This patch instates
the output filters in 3 cases where sub-requests are/may in fact
be used as the real request later on.

This is a relatively risky change (which should not be back-ported
without further discussion) and may break caches in combination
with internal redirects/vary/negotiation in subtle ways.

See the thread starting at [1] and in particular the general
concerns of rpluem at [2] with respect to sub requests
and (fast_)internal redirects possibly needing a more
thorough overhaul.

1: http://mail-archives.apache.org/mod_mbox/httpd-dev/200802.mbox/ajax/%3c335D1A4B-25E2-4FF1-8CDF-5010A7FBD293@webweaving.org%3e
2: http://mail-archives.apache.org/mod_mbox/httpd-dev/200802.mbox/%3c47ACE1D4.4060702@apache.org%3e

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@620133 13f79535-47bb-0310-9956-ffa450edef68