objects.base: fixed object.delete()

objects.base: fixed object.delete()

It was using wrong call to delete an object from a session.

Also, expanded test_create() test to check all basic operations:
get_by_id, create, update, and delete.

To facilitate update() check, introduced a new field for objects called
fields_to_update that stores names of fields that are not expected to be
updated in an object. This allows us to keep the above mentioned test
untangled from specific object type.

Also made get_by_id() behave correctly (returning None) if the object
does not exist.

Change-Id: I1aecb2e7c4d8cb8f239072d1cb9df3db29dcedde