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, ...).
In this case the abstract base class doesn't add anything, since there are only two implementations and one is an extension of the other. The super call would make it straight forward to add functionality for all rpc methods (e.g. logging).