public void Implement_interface()
        {
            var sut = new TaxRateStorage();

            Assert.That(sut, Is.InstanceOf <IStoreTaxRate>());
        }
        public void Default_rate_is_20()
        {
            var sut = new TaxRateStorage();

            Assert.That(sut.CurrentRate, Is.EqualTo(20M));
        }