Headers are not passed to Postal Message

[a47ae]

Hey, maybe I setup something wrong, but with the logEmailAgainstModel method set and the 'postal.enable.emaillogging' value set to true I do not get the notifiable_class and notifiable_id headers passed to Postal.

On inspection they do get set on the underlying Swift_Message instance, but get lost during the transformation to a Postal message. If I understood the code correctly the swiftToPostal method of the PostalTransportClass simply does not set any custom headers.

This does not only break the email logging feature, but also does not allow the application to pass any additional headers, which could break some workflows.