[ORM] Update should not update columns which did not changed

[bbankowski]

e.g.

$product = Products::findById(1);
$product->name = 'test';
$product->update();

Should update name column only. Right now it will update everything.

[bbankowski]

Important:

• before and after callback are invoked regardless of update (e.g. if update is not invoked because nothing changed, callbacks are still invoked) - needs to be added as note to docs

More tests would be nice, especially with updateAttributes.

[woru]

Why do we need _trackUpdates?

[bbankowski]

Removed.

[woru]

unsuccessful insert resets _updatedAttributes update does not reset _updatedAttributes probably unsuccessful update should not reset _updatedAttributes Why do we reset _updatedAttributes in Model::newInstance?