Overriding the rest# serialiser

RestSharpXmlSerializer

 public class RestSharpXmlSerializer : RestSharp.Serializers.ISerializer
        {

            public string Serialize(object obj)
            {
                string retval = null;
                if (obj != null)
                {
                    var sb = new StringBuilder();
                    using (var writer = XmlWriter.Create(sb, new XmlWriterSettings() { OmitXmlDeclaration = true }))
                    {
                        new System.Xml.Serialization.XmlSerializer(obj.GetType()).Serialize(writer, obj);
                    }
                    retval = sb.ToString();
                }
                return retval;
            }

            public string ContentType { get; set; }

            public string DateFormat { get; set; }

            public string Namespace { get; set; }

            public string RootElement { get; set; }
        }

Using it

 private static RestRequest CreateRequest(string resource)
        {
            var request = new RestRequest
                              {
                                  Resource = resource,
                                  XmlSerializer =
                                      new RestSharpXmlSerializer
                                          {ContentType = "application/xml", DateFormat = DateFormat.RoundTrip}
                              };

            return request;
        }