Unexpected RST message with observations when first response use block2.

[sbernard31]

When you send an observation request, if the device answer to the request with block1, after "response timeout" milliseconds the observation will be canceled and device will received RST message for the next notification.

The issue seems to be about CoapAsyncRequestObserver.

A task to clean request in case we never get a response is create on onReadyToSend.
But if transparent block2 is used for the response of the observe request, this method is called for each block2 request and so if there is n block we will create n task. But the current code expect there is only 1 task and when response is received we cancel only 1 task. So the n-1 others tasks will cancel the request and so remove the observation.

I will prepare a fix about that.