BUM-2705 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.
+1, this is something we had problems with in the JS implementation of this that I sent to you @konrad-intenthq