public void Test_EnablingOpenTracking()
        {
            var mail = BasicMailBuilder
                       .EnableOpenTracking()
                       .Build();

            var message = new SendGridMessage();

            message.EnableOpenTracking();
            Assert.IsFalse(string.IsNullOrEmpty(message.Header.JsonString()));
            Assert.AreEqual(message.Header.JsonString(), mail.Header.JsonString());
        }