Update Server-Side SDKs To Support Session ID Response + tests.

Signed and unsigned calls are wrapped in an associative array containing two elements:
   - result_obj: An associative array representing the deserialised JSON response.
   - session_key: valid session key for a user.
Actually, as discussed, probably `body` is nicer, as (you quite rightly pointed out) it avoids a possible `$result['result']` double naming confusingness. :-)