er-coap (observe): fix possible duplicate value for observe option in GET response and subsequent notification

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).
Merge pull request #778 from alessandrelli/fix_duplicate_observe_value

er-coap (observe): fix possible duplicate value for observe option