Using WebFaultException from IErrorHandler

Question

If I derive all of my exceptions from WebFaultException, and throw them directly from my services, the StatusCode set in the WebFaultException gets passed to the client. I'd rather not have to do that, since I'd rather throw generic exceptions like NotImplementedException, from my code.

I've set up an IErrorHandler, with the expectation that I'd be able to catch exceptions like NotImplementedException, and then just do the conversion to a WebFaultException there.

My code looks like this:

public class HTTPBindingErrorHandler: IErrorHandler
{
    public void ProvideFault(Exception exception, System.ServiceModel.Channels.MessageVersion version, ref System.ServiceModel.Channels.Message fault)
    {
        if (exception is NotImplementedException)
        {
            WebFaultException wfe = new WebFaultException(HttpStatusCode.NotImplemented);
            MessageFault mf = wfe.CreateMessageFault();
            fault = Message.CreateMessage(version, mf, wfe.Action);
        }
    }

    public bool HandleError(Exception exception)
    {
        return false;
    }
}

When stepping through, I see that I'm getting to the code in ProvideFault, but the Status returned to the client is 400, not 501. Additionally, the body is:

<Fault
    xmlns="http://schemas.microsoft.com/ws/2005/05/envelope/none">
    <Code>
        <Value>Receiver</Value>
        <Subcode>
            <Value
                xmlns:a="http://schemas.microsoft.com/2009/WebFault">a:NotImplemented
            </Value>
        </Subcode>
    </Code>
    <Reason>
        <Text xml:lang="en-US">Not Implemented</Text>
    </Reason>
</Fault>

1. Why isn't the correct status code being returned?
2. Why isn't the body in json?

Addition: I've tried the recommendation of setting AutomaticFormatSelectionEnabled = false to the webhttpbehavior, but it still does not work. I'm self hosting, but I don't think that should make the difference. Here is the code I use to set up the service:

Uri newUri = new Uri(info.baseURI, info.Path);
WebServiceHost host = new WebServiceHost(info.Type, newUri);
WebHttpBinding binding = new WebHttpBinding(WebHttpSecurityMode.TransportCredentialOnly);
binding.Security.Transport.ClientCredentialType = HttpClientCredentialType.InheritedFromHost;


ServiceEndpoint ep = host.AddServiceEndpoint(info.Interface, binding, newUri);
ep.Behaviors.Add(new WebHttpBehavior() { 
    DefaultOutgoingResponseFormat = WebMessageFormat.Json, 
    DefaultBodyStyle = WebMessageBodyStyle.Wrapped,
    AutomaticFormatSelectionEnabled = false } );
ep.Behaviors.Add(new WebHttpCors.CorsSupportBehavior());
ep.Behaviors.Add(new HTTPBindingErrorBehavior());