We set up vads_trans_id and theme according to payzen format.

        If fields values are explicitely set by user, we do not override
        their values.

def save(self):
        """
        We set up vads_trans_id and theme according to payzen format.

        If fields values are explicitely set by user, we do not override
        their values.
        """
        if not self.vads_trans_date:
            self.vads_trans_date = datetime.datetime.utcnow().replace(
                tzinfo=utc).strftime("%Y%m%d%H%M%S")
        if not self.vads_trans_id:
            self.vads_trans_id = tools.get_vads_trans_id(
                self.vads_site_id, self.vads_trans_date)
        if self.theme and not self.vads_theme_config:
            self.vads_theme_config = str(self.theme)
        if not self.pk:
            super(PaymentRequest, self).save()
        self.set_vads_payment_config()
        self.set_signature()
        super(PaymentRequest, self).save()