Fix for adding an undefined XML namespace to the generated XML's child namespace
Allowing request.xml and response.json to be optional for tests.
Allowing response.xml to be optional in tests.
Added the possibility to work with `wsdl` files which pull their `schema`s in through `import`s and declare their message `part` to have an `element` attribute instead of `type`.

This fix should prevent the library from failing on an `undefined` `input.$type` property when creating a new SOAP message.