Fix subreqs with non-defined Content-Types being served improperly.

Fix subreqs with non-defined Content-Types being served improperly.

If we do not know a C-T for a subreq, we *must* propogate that
non-knowledge upwards to the main request.

Previously, if you used a DirectoryIndex with a file without a C-T (say
.shtml without AddType), the r->content_type will be kept as
httpd/unix-directory when we promoted the subreq in mod_dir.  Since there
would be no handler on this file, ap_invoke_handler (config.c:355) would
set the handler to be httpd/unix-directory (which was the old C-T of the
dir).  This would then trigger the handler to become httpd/unix-directory.
mod_autoindex would then try to serve the request.  But, the filename
was propogated upwards by mod_dir's DirectoryIndex via
internal_fast_redirect - it would then return a 403 trying to generate a
mod_autoindex page for a file.

Now, we will use ap_default_type() which is correct.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94676 13f79535-47bb-0310-9956-ffa450edef68