XmlSerializer.Serialize() как поменять кодировку с UTF-16 на UTF-8

        // VALIDATE
        XmlSerializer serializer = new XmlSerializer(typeof(AMUpdateAddressRequestType));
        AMUpdateAddressRequestType request = serializer.Deserialize(new StringReader(requestXML)) as AMUpdateAddressRequestType;
        // SERIALIZE
        TextWriter writer = new StringWriter();
//      writer.Encoding = new UTF8Encoding();
//      XmlTextWriter xmlWriter = new XmlTextWriter(writer, new UTF8Encoding());
        serializer.Serialize(writer, request );
        requestXML = writer.ToString();
        // CALL
        log.Debug('>>> ' + requestXML );
        string resultXML = service.updateAddress(requestXML);       
        log.Debug('<<< ' + resultXML );
        //
        Assert.IsNotNull(resultXML);

   // MEMORY STREAM 
   MemoryStream stream = new MemoryStream(); 
   XmlWriter xml = new XmlTextWriter(stream, new UTF8Encoding() ); 
   serializer.Serialize(xml, request ); 
   stream.Seek(0, SeekOrigin.Begin); 
   StreamReader reader = new StreamReader(stream); 
   // GET STRING 
   string requestXML = reader.ReadToEnd();