cancellation method
Create a cancellation request
Implementation
@internal
CoapRequest cancellation() => CoapRequest.get(_request.uri)
// Copy options, but set Observe to cancel
..setOptions(_request.getAllOptions())
..observe = ObserveRegistration.deregister.value
// Use same Token
..token = _request.token
..destination = _request.destination
// Dispatch final response to the same message observers
..copyEventHandler(_request);