HtmlContent property will set value, does not get value

[JoshuaCarroll]

Steps to reproduce:

1. Create a SendGridMessage

    SendGridMessage message= MailHelper.CreateSingleEmail(
        new EmailAddress("from@email.com"),
        new EmailAddress("to@email.com"),
        "This is a subject",
        "This is plaintext content",
        "This is HTML content"
    );

2. Read the value of the message.HtmlContent. It should have the string "This is HTML content" however it returns null.

The summary of the HtmlContent property says, "Gets or sets a Content object with a Mime Type of text/html."