API CHANGE: DataObject's internal $this->record array doesn't import null values, so that they don't get written back out.
API CHANGE: DataObject queries explicitly list columns, rather than using *. This means that extraneous columns won't be included.
MINOR: Updated tests for db abstraction.
git-svn-id: svn://svn.silverstripe.com/silverstripe/open/modules/sapphire/trunk@76372 467b73ca-7a2a-4603-9d3b-597d59a354a9