Changes

Avoid NPE when processing response message in unit tests

The CoapAsyncRequestObserver spawns a cleaning task when an outbound
message is ready to be sent. When invoking the onResponse method in a
unit test to trigger the processing of an incoming response, a NPE is
thrown if onReadyToSend has not been invoked before.

All other methods use the private cancelCleaningTask method to stop the
cleaning task which performs a non-null check before stopping the task.
The onResponse method has been changed to also use this method in order
to prevent the NPE.

Signed-off-by: Kai Hudalla <kai.hudalla@bosch.io>