Fix calling (raw_)rpc wrapped methods

Make it possible to call methods wrapped with @rpc and @raw_rpc as
any normal methods, by keeping the normal behaviour of Python methods
and any RPC call must be by invoking method.rpc_call(channel, ...).
I always default to private (internal) unless there is a need to make it public, b/c at soon as something go public there is no coming back :)