def build_invoice(invoice = {})
      case invoice
        when Invoice then     invoice.gateway = self
        when Hash then        invoice = Invoice.new(invoice.merge(:gateway => self))
      end
      invoice
    end
def build_invoice(invoice = {})
      case invoice
        when Invoice then     invoice.gateway = self
        when Hash then        invoice = Invoice.new(invoice.merge(:gateway => self))
      end
      invoice
    end
Factory method for building new Invoice objects associated with this gateway.