client generation with dotnet-svcutil: usage of DataTable in Service-Interface affects serialization of guid as string

**Describe the bug**
We provide a wcf-service with several methods with guid-parameters and guid return types. When using dotnet-svcutil to create a client everything works like expected.

After adding a method which returns a `System.Data.DataTable` all other generated client-methods changed. Every usage of `Guid` was replaced by `string`.

`dotnet-svcutil` generated a client with `string`-parameters instead of `guid `parameters when a method with `DataTable`-returntype is present.

**To Reproduce**

Service which works correctly

```
public class Service1 : IService1
{
public int GetValue(Guid valueId)
{
throw new NotImplementedException();
}

public string GetTable(Guid section)
{
throw new NotImplementedException();
}
}
```

WSDL of service without datatable
```

```

Generated client interface (with expected guid parameters):
```

[System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Tools.ServiceModel.Svcutil", "2.0.3")]
[System.ServiceModel.ServiceContractAttribute(ConfigurationName="IService1")]
public interface IService1
{

[System.ServiceModel.OperationContractAttribute(Action="http://tempuri.org/IService1/GetValue", ReplyAction="http://tempuri.org/IService1/GetValueResponse")]
System.Threading.Tasks.Task GetValueAsync(System.Guid valueId);

[System.ServiceModel.OperationContractAttribute(Action="http://tempuri.org/IService1/GetTable", ReplyAction="http://tempuri.org/IService1/GetTableResponse")]
System.Threading.Tasks.Task GetTableAsync(System.Guid section);
}

```

Now changing return type of GetDataTable(...)

```
public class Service1 : IService1
{
public int GetValue(Guid valueId)
{
throw new NotImplementedException();
}

public DataTable GetTable(Guid section)
{
throw new NotImplementedException();
}
}
```

the generated wsdl is as follows:

```

```

when now generating a client with dotnet-svcutil this client interface becomes generated:
The parameter type of GetValueAsync is now string but should be guid.
```

[System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Tools.ServiceModel.Svcutil", "2.0.3")]
[System.ServiceModel.ServiceContractAttribute(ConfigurationName="IService1")]
public interface IService1
{

[System.ServiceModel.OperationContractAttribute(Action="http://tempuri.org/IService1/GetValue", ReplyAction="http://tempuri.org/IService1/GetValueResponse")]
[System.ServiceModel.XmlSerializerFormatAttribute(SupportFaults=true)]
System.Threading.Tasks.Task GetValueAsync(string valueId);

[System.ServiceModel.OperationContractAttribute(Action="http://tempuri.org/IService1/GetTable", ReplyAction="http://tempuri.org/IService1/GetTableResponse")]
[System.ServiceModel.XmlSerializerFormatAttribute(SupportFaults=true)]
System.Threading.Tasks.Task GetTableAsync(GetTableRequest request);
}
```

**Expected behavior**
I expected that the usage of DataTable as return type has no side effects to the signature of other methods.