Add method to permit AC handler to expect keyhash

Add method to permit AC handler to expect keyhash

Add an expects_keyhash method to
OpenILS::WWW::AddedContent::Syndetic and look for it from
OpenILS::WWW::AddedContent. This allows us to pass a keyhash
(containing isbn/upc/whatever) to this handler while still passing a
simple scalar containing an ISBN to other AddedContent handlers.

Signed-off-by: Jeff Godin <jgodin@tadl.org>