Mark fields changed only when value is different

Currently `Jam_Validated` put a field in the `_changed` array whenever a value
is set. It doesn't matter if the value being set is a new one or the same as
before.

This is a limitation of the associations and `Jam_Model`. It is hard to know
if the association has changed.

But it is very easy for fields.

This involves changing tests and validation accordingly

The model tests so far did not test things logically.

The tests for the Sortable behavior started with some strange fixture data and
therefore some tests regarding the position field were affected.