Add the AWS-generated message_id to the ruby message object
Add suffix to message_id
Make sure raw_message responds to message_id before setting it
Merge branch 'master' into add-message-id-to-message
Merge branch 'master' into add-message-id-to-message
Added test for setting message id on raw messages.

When calling `AWS::SimpleEmailService#send_raw_message` with an
object that responds to `#message_id=`, then the message ID
from the SES response will be set on the raw message.

Added a missing test and corrected the `#responds_to?` check.

Fixes #261