How do I serialize an enum value as an int?

I want to serialize my enum-value as an int, but i only get the name.

Here is my (sample) class and enum:

public class Request {
    public RequestType request;
}

public enum RequestType
{
    Booking = 1,
    Confirmation = 2,
    PreBooking = 4,
    PreBookingConfirmation = 5,
    BookingStatus = 6
}

And the code (just to be sure i'm not doing it wrong)

Request req = new Request();
req.request = RequestType.Confirmation;
XmlSerializer xml = new XmlSerializer(req.GetType());
StringWriter writer = new StringWriter();
xml.Serialize(writer, req);
textBox1.Text = writer.ToString();

This answer (to another question) seems to indicate that enums should serialize to ints as default, but it doesn't seem to do that. Here is my output:

<?xml version="1.0" encoding="utf-16"?>
<Request xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <request>Confirmation</request>
</Request>

I have been able to serialize as the value by putting an "[XmlEnum("X")]" attribute on every value, but this just seems wrong.