quick SimpleACK fix

[Tchuba]

Proposed way to fix the issue with WriteProperty request - timeout exception is thrown in ServiceFutureImpl here:

 if(ex == null && ack == null && fail == null)
    ex = new BACnetException("Timeout waiting for response.");

even though ACK was received. SimpleAckService is just a stump and should be expanded if necessary - it's only purpose is for something to be passed instead of null in DefaultTransport to the success method