fixes incorrect JSONArray/String usage in ServerRpcHandler (#14471)

As described by Johannes, the RPCRequest constructor first takes whole
JSON as string and builds its attribute json from it (by parsing).
Then it reads from this parsed JSON the array called "rpc".
However it previously did a .toString on it only to re-parse it.
Newer json versions complain that "rpc" is not a string, but an array.

Change-Id: If4bb45abad551b183d757f0aa326c37c070ce103