CoAP trigger: discovery message type

[Adirio]

The discovery response message type is being send with both the same type and id as the request:

178	res := &coap.Message{
179		Type:      msg.Type,
180		Code:      coap.Content,
181		MessageID: msg.MessageID,
182		Token:     msg.Token,
183		Payload:   []byte(payloadStr),
184	}

There are two posibilities a confirmable (CON) and a non-confirmable (NON) request:

1. CON: an acknowledgment (ACK) is required. The response can either be piggybacked in the ACK or in another message. Piggybacked is suggested when able and as discovery is straightforward, it would be as easy as changing the message type:

res := &coap.Message{
	Type:      coap.Acknowledgment,
	Code:      coap.Content,
	MessageID: msg.MessageID,
	Token:     msg.Token,
	Payload:   []byte(payloadStr),
}

2. NON: the response can not be given as an ACK according to the spec. Using the same response type is suggested, but not enforced by the RFC. Message IDs are only shared with their corresponding ACKs, which is not the case. A new ID should be used, maybe randomly generated:

res := &coap.Message{
	Type:      coap.NonConfirmable,
	Code:      coap.Content,
	MessageID: generateNewId(),
	Token:     msg.Token,
	Payload:   []byte(payloadStr),
}

[Adirio]

I would apreciate some feedback. I can submit a pull request myself but will not develop anything until one of the developers approves it.