refactor #determine_order_user helper method
refactor #determine_order_user helper method

There's no behavior change to this. It allows subclasses
to easily override how to determine the user.

scottcrawford03/solidus,bonobos/solidus,scottcrawford03/solidus,scottcrawford03/solidus,bonobos/solidus,bonobos/solidus,scottcrawford03/solidus,bonobos/solidus