Change object.rename() override option to not delete existing if nothing match

[hueniverse]

Previously, this:

const schema = Joi.object().rename('a', 'b', { override: true });
const object = { b: 1 };

Would result in b being removed from the object because no a was found. This is unexpected behavior. If there is a real use case for this, we need another flag that means "only allow b from the rename, otherwise remove" which is by itself very odd. A better logic would be to forbid b unless it is from the rename.

Either way, I am changing this and if there is real need for the existing behavior, we can look into the proper way to support it.