removed the assumption that unsubscription request always come from the MailChimp webhook. this will allow go-mailchimp to unsubscribe user as a result of setting the do_not_email user profile to TRUE

related to https://github.com/GigaOM/legacy-pro/issues/3858
Merge pull request #7 from wluo/github-3858

handle do_not_email user profile property change in a more generic way