fix soap execute & parameters

It seems the `__soapCall` method expects the parameters to be wrapped in an array with 'parameters' as its key. It's mentioned in the comments at http://php.net/manual/en/soapclient.soapcall.php

With this update, the Soap service at http://ec.europa.eu/taxation_customs/vies/checkVatService.wsdl worked for me.