Bugfix: timestamp as 32bit integer (#916)

32bit signed integer will overflow in year 2038

Author: matrix0123456789

src/SendGrid/Helpers/Mail/SendGridMessage.cs

@@ -387,7 +387,7 @@ public void AddCustomArgs(Dictionary<string, string> customArgs, int personaliza
         /// <param name="sendAt">Specify the unix timestamp for when you want the email to be sent from Twilio SendGrid.</param>
         /// <param name="personalizationIndex">Specify the index of the Personalization object where you want to add the send at timestamp.</param>
         /// <param name="personalization">A personalization object to append to the message.</param>
-        public void SetSendAt(int sendAt, int personalizationIndex = 0, Personalization personalization = null)
+        public void SetSendAt(long sendAt, int personalizationIndex = 0, Personalization personalization = null)
         {
             personalization = GetPersonalization(personalizationIndex, personalization);
             personalization.SendAt = sendAt;