- session.merge() will not expire existing scalar attributes

- session.merge() will not expire existing scalar attributes
on an existing target if the target has a value for that
attribute, even if the incoming merged doesn't have
a value for the attribute.  This prevents unnecessary loads
on existing items.  Will still mark the attr as expired
if the destination doesn't have the attr, though, which
fulfills some contracts of deferred cols.  [ticket:1681]

- session.merge() works with relations that specifically
don't include "merge" in their cascade options - the target
is ignored completely.