changed attribute name from nazwa to name in ShippingMethod

This resolves the error:

{"OpenPayU":{"OrderCreateResponse":{"Status":{"StatusCode":"ERROR_VALUE_INVALID","StatusDesc":"The
content of element 'ShippingMethod' is not complete. One of
'{\"http://www.openpayu.com/20/openpayu.xsd\":Name}' is
expected.","Location":"OpenPayU.OrderCreateRequest.OrderCreateRequest$ShippingMethods.ShippingMethodType"},"ResId":"{f8773afd-4e2f-405b-a131-567d8895f430}"}}}.

That appears when creating the request object.