When a client sends a CoAP request with no block2 size,
the default one would be set to REST_MAX_CHUNK_SIZE.
However, this is not guaranteed to be a power of 2.
This can lead to clients receiving a bigger payload than expected as
part of the header, and ending up with duplicated content.
Setting the default to COAP_MAX_BLOCK_SIZE,
which is guaranteed to be a power of 2, fixes this.
Currently, the observe value for a response to a GET observe request is always set to zero. That may cause the subsequent notification to have the same observe value. In fact, that happens every time an observable resource is observed for the first time (since the obs_counter is implicitly initialized to zero).
This patch fixes such a problem by setting the observe option value of responses to obs_counter (and then incrementing it).
