- Added checks inside the UOW to detect the unusual

- Added checks inside the UOW to detect the unusual
condition of being asked to UPDATE or DELETE
on a primary key value that contains NULL
in it.  [ticket:2127]

- Some refinements to attribute history.  More
changes are pending possibly in 0.8, but
for now history has been modified such that
scalar history doesn't have a "side effect"
of populating None for a non-present value.
This allows a slightly better ability to
distinguish between a None set and no actual
change, affects [ticket:2127] as well.

- rewriting the history tests in test_attributes to be
individual per operation/assertion.   its a huge job
so this is partial for the moment.