header.

Until now, we have not allowed IMS on objects without LM header but
after due consideration of our role as web-server, that restriction is
found too hard:  Varnish will, by definition, not find and object which
is not valid, so we can trust the time we put it into the cache to
be the LM date.

But we can not synthesize a LM header based on this, as this would allow
down-stream client-side caches to make unwarranted decisions (see RFC2616
13.3.4 p88)

Fixes: #795



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5505 d4fa192b-c00b-0410-8231-f00ffab90ce4