Add message id to message

The ruby mail gem [generates its own MessageId header](https://github.com/mikel/mail#or-do-worry-about-message-ids), unfortunately AWS overrides that when sending mail.

I've made a change that sets the `message_id` field on the mail message object after the response from the AWS API comes back when calling `deliver`.

Many applications rely on having a valid `message_id` to tie together emails with responses, so this is an important change.