Use InvariantCulture for XmlSerializer

There is a mismatch between Json and Xml serialization, with regards to CultureInfo. The SimpleJson serializer, takes care of serializing with CultureInfo.InvariantCulture:

https://github.com/restsharp/RestSharp/blob/master/RestSharp/SimpleJson.cs#L1112

This pull requests makes the same changes for the XmlSerializer.

This problem occured for me when working with decimals in a culture where comma and not dot was used for decimal separater.