public void Test_EnablingGravatar()
        {
            var mail = BasicMailBuilder
                       .EnableGravatar()
                       .Build();

            var message = new SendGridMessage();

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