public void Calculate_The_Correct_Result()
        {
            var sut = new LuhnChecksumCalculator();

            sut.Calculate("7992739871").Should().Be(3);
            sut.Calculate("123456789").Should().Be(7);
        }
        public void When_Subject_Is_Null_Throw_ArgumentNullException()
        {
            var sut = new LuhnChecksumCalculator();

            Action action = () => sut.Calculate(null);

            action.Should().Throw <ArgumentNullException>();
        }