That one is simple. Since you know that you used your signal, have the cop check your signals while he is there. It is unlikely that he will write a failure to signal citation because it won't stand up in court. If he is a dick and still writes the ticket, you replace the bulb and show up at court with the receipt and a witness. The judge will have no choice but to reduce the charge from failure to signal to improper equipment, maybe even dismiss the charge entirely. Now you have nothing to pay for, except maybe to buy your buddy a beer.