ASP.NET Web API does not return XML

I have a controller and a method that adds the user to the database.

I call this from Fiddler the request header as follows:

Content-Type: application / xml

Accept: application / xml

Host: localhost: 62236

Content-Length: 39

And the request body is

<User>
  <Firstname>John</Firstname>
  <Lastname>Doe</Lastname>
</User>

This works, as expected, the method is called, the user object is processed in the PostUser method.

 public class UserController : ApiController
    {
        public HttpResponseMessage PostUser(User user)
        {
            // Add user to DB
            var response = new HttpResponseMessage(HttpStatusCode.Created);
            var relativePath = "/api/user/" + user.UserID;
            response.Headers.Location = new Uri(Request.RequestUri, relativePath);
            return response;
        }
    }

I am doing model validation in my class

public class ModelValidationFilterAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(HttpActionContext actionContext)
    {
        if (actionContext.ModelState.IsValid == false)
        {
            // Return the validation errors in the response body.
            var errors = new Dictionary<string, IEnumerable<string>>();
            foreach (KeyValuePair<string, ModelState> keyValue in actionContext.ModelState)
            {
                errors[keyValue.Key] = keyValue.Value.Errors.Select(e => e.ErrorMessage);
            }

            actionContext.Response =
                actionContext.Request.CreateResponse(HttpStatusCode.BadRequest, errors);
        }
    }
}

BUT if I publish the following

<User>
  <Firstname></Firstname> **//MISSING FIRST NAME**
  <Lastname>Doe</Lastname>
</User>

The model is invalid and a JSON response is returned, although I said Accept: application / xml.

If I perform model validation in UserController, I get the correct XML response, but when I execute it in ModelValidationFilterAttribute, I get JSON.