This adds explicit tests for the returned HTTP status codes to
interface_rpc.py (for error cases) and the HTTP JSON-RPC client in
general for success.
PR 15381 brought up discussion about the HTTP status codes in general,
and the general opinion was that the current choice may not be ideal
but should not be changed to preserve compatibility with existing
JSON-RPC clients. Thus it makes sense to actually test the current
status to ensure this desired compatibility is not broken accidentally.
Split off AuthServiceProxy.get_request method to make it easier to batch RPC
requests without duplicating code and remove leading underscore from _batch
method.
This does not change any existing behavior.
