public void Test_AddHeaders()
{
var mail = BasicMailBuilder
.AddHeaders(new Dictionary <string, string>()
{
{ "X-TEST", "MAILBUILDER" },
{ "X-TEST-2", "SENDGRID" }
})
.Build();
var message = new SendGridMessage();
message.Headers.Add("X-TEST", "MAILBUILDER");
message.Headers.Add("X-TEST-2", "SENDGRID");
Assert.IsTrue(mail.Headers.All(e => message.Headers.Contains(e)));
}