Last-Modified and caching header not sent on GET/HEAD

While the dav spec does not require it (the language actually reads in
the oppposite direction: if you send a Last-Modified header you must
include a getlastmodified prop), there is no point not sending the
header since we already have the information and do provide it in
PROPFIND requests.

Yes, i know mod_dav_svn spits out other headers that are better for
caching, but a lot of people are still using squid, which is an
HTTP/1.0 proxy unfortunately.  It *never* caches svn requests:

X-Cache: MISS from dethstar.pretzelnet.org

Like i said, we already have the info, this comes practically free.

Original issue reported by epg