Fix "AssertionError: Invalid child type" for WSDLs containing imports and includes

Any WSDL containing imports or includes will cause an "AssertionError: Invalid child type" Example:
```xml
...
<wsdl:import location="https://192.168.2.2:8080/soap/Konto?wsdl=KontoService.wsdl"
namespace="http://service.soap.hbci.jameica.willuhn.de/"></wsdl:import>
...
```

This issue has been waiting for a year => https://github.com/vpulim/node-soap/issues/125

I suggest those types of elements are ignored temporary until node-soap has some logic to handle them.