connected service doesn't handle nillable date's (works on full framework - old known .NET FX issue?).

i have a old legacy "Service Reference" style wcf client that is working with this wsdl correctly (i made sure it was updated for the current published wsdl)
and when trying to convert all the old legacy stuff to the new "Connected Services" it fails to properly handle because the wsdl has Nullable's inside (thanks java....)

wsdl https://ws.tradetracker.com/soap/merchant?wsdl

**repo code**
```
// setup the client
var soapClient = new MerchantInterfaceClient(
new BasicHttpBinding(BasicHttpSecurityMode.Transport) { AllowCookies = true, MaxReceivedMessageSize = 1024 * 10 * 10 * 10 },
new EndpointAddress(new Uri("https://ws.tradetracker.com/soap/merchant")));

// replace "nl_NL" with "Locale.nl_NL" when using the old net461 wcf service reference
var tsk = soapClient.authenticateAsync("123456", "12345abcdefghijklmnopqrstuvwxyz123456789", true, "nl_NL", false);
tsk.Wait();

```
**expected result** (because these credentials are fake of course)

> FaultException: Failed to authenticate.

**actual result**
`System.InvalidOperationException: ''date' is an invalid value for the SoapElementAttribute.DataType property. The property may only be specified for primitive types.'`

netstandard2.0, visual studio 2017 (with the latest updates, etc etc)