Client/Proxy: use keyword arguments to match the correct order

The XMLRPCTransport __init__ method defines several arguments with default
values. The current call missed the use_datetime argument, so that the
client_timeout will set the use_datetime paramenter and the protocol option
was used as timeout (does not work, because a float is required and raises
an exception).
Merge pull request #218 from AlexanderS/fix-arguments

Client/Proxy: use keyword arguments to match the correct order