introduce Client.methodCallWithTransport

[blaenk]

I had forgotten that I had contributed to this project many years ago :smile:

I really need to be able to perform requests via SCGI instead of HTTP. I was originally doing this by explicitly requiring xmlrpc/lib/Client, Deserializer, and so on to attach my own scgiMethodCall to the Client's prototype, but I figured that this might be a cleaner way.

Essentially changes Client.methodCall to take a specific transport object which is used to perform the request. This enables one to use a transport other than HTTP or HTTPS, for example, SCGI.

The original Client.methodCall is now implemented in terms of this new method.