Fix case where an included file may change but the shtml file has not -

Fix case where an included file may change but the shtml file has not -
since we returned Last-Modified/ETag headers, we said that it was possibly
cacheable.  Wrong.

See RFC 2616 13.3.4 for more details (these are SHOULD/MAY clauses, so I
think we can discard them when we are dealing with dynamic data).

(Justin cleaned up the comment to cite RFC 2616.)

Submitted by:	Ian Holsman <ianh@cnet.com>
Reviewed by:	Justin Erenkrantz, Aaron Bannert, Brian Pane

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91080 13f79535-47bb-0310-9956-ffa450edef68