public void ReturnsEmptyStringWhenGivenShorterThan4()
        {
            var expected = String.Empty;
            var input    = "1234";

            var ms = new Masker();

            var actual = ms.MaskSecureNumbers(input);

            Assert.AreEqual(expected, actual);
        }
        public void ReturnsMaskedString()
        {
            var expected = "#2345";
            var input    = "12345";

            var ms = new Masker();

            var actual = ms.MaskSecureNumbers(input);

            Assert.AreEqual(expected, actual);
        }