public void Test_IncludeUniqueArg()
        {
            var mail = BasicMailBuilder
                       .IncludeUniqueArg("key", "value")
                       .Build();

            var message = new SendGridMessage();

            message.AddUniqueArgs(new Dictionary <string, string>()
            {
                { "key", "value" }
            });
            Assert.IsFalse(string.IsNullOrEmpty(message.Header.JsonString()));
            Assert.AreEqual(message.Header.JsonString(), mail.Header.JsonString());
        }