Fix email template creation date not being persisted

The database schema from https://github.com/magento/magento2/blob/develop/app/code/Magento/Email/sql/email_setup/install-2.0.0.php#L62-L67 uses the column `added_at` for storing the date the email template creation date. When `setCreatedAt` is used, that date is not persisted to the database, leaving the column `NULL` in the database.