public void TestAddSubVal()
        {
            var substitutionStrings = new List<String> {"foo", "bar", "beer"};
            var mock = new Mock<IHeader>();

            var sg = new SendGrid(mock.Object);
            sg.AddSubVal("-name-", substitutionStrings);

            mock.Verify(foo => foo.AddSubVal("-name-", substitutionStrings));
        }