RestSharp.Serializers.XmlSerializer: Preserve XML namespace for lists.

[patrodyne]

To preserve the parent XML namespace for List instances, during XML serialization...

In two methods of RestSharp.Serializers.XmlSerializer:

1. public string Serialize(object obj),
2. private void Map(XElement root, object obj).

Change

    var instance = new XElement(itemTypeName);

To

    var instance = new XElement(itemTypeName.AsNamespaced(Namespace));