Allow soap.createClient to create a new SOAP client from a WSDL string

[ckuijjer]

Currently soap.createClient can create a new SOAP client from a WSDL url or local filesystem path. Creating a new SOAP client from a string containing the WSDL isn't supported.

This PR extends the open_wsdl function to detect a XML string being passed to it, and if so directly creates a WSDL based on it (instead of first loading the definition from file or uri)