Maintain 'source_class' of the param across a copy

When generating the SOAP Response the type of a return complexType wasn't being included, this meant the response didn't match the WSDL specification.

Maintaining the 'source_class' when copying a param allows the response XML to include the correct type for that message part.
Merge pull request #131 from meanphil/master

Maintain 'source_class' of the param across a copy