HEAD and max-age=0

This comment:

* RFC 2068 seems to indicate there is no "conditional HEAD"
* request. We cannot validate a cached object for a HEAD
* request, nor can we return 304.

has me stumped. max-age=foo should apply to HEAD AFAICT. We may not be
able to perform IMS processing, but we should be able to validate and
return the upstream object if needed.

Can anyone point me with a little more precision than 'rfc 2068' at the
logic the comment was drawn from?

Rob