enhancement: questionable default value for params

[kroeplin]

The methods 'request' and 'notify' of the class 'RPCClient' in 'rpc.py' use an empty array as default value for 'params'.

The specification states: "This member MAY be omitted."

With a default value of 'None' the following code (with ugly repetition?) would send better requests without parameters:

        if params is None:
            self.writer.write_value({'jsonrpc': '2.0', 'method': method, 'id': request_id})
        else:
            self.writer.write_value({'jsonrpc': '2.0', 'method': method, 'params': params, 'id': request_id})

and

        if params is None:
            self.writer.write_value({'method': method})
        else:
            self.writer.write_value({'method': method, 'params': params})

(By the way: the 'jsonrpc': '2.0' part is currently missing for notifications.)