how to send/create status code as response for Unauthorized requests in .Net Framework 4.0

Question

I am using .NET framework 4.0 with WEB API 1.0. I am trying to send an HTTP 401 Unauthorized response for an unauthorized API request, but every time I receive a status code 200 OK. In the response body I get:

StatusCode: 401, ReasonPhrase: 'Unauthorized', Version: 1.1, Content: System.Net.Http.StringContent, Headers:
{
    Content-Type: text/plain; charset=utf-8
}

Below is my code:

[HttpPost]
public HttpResponseMessage GetShopredact()
{
    var response = new HttpResponseMessage(HttpStatusCode.Unauthorized)
    {
        Content = new StringContent("Invalid Request", System.Text.Encoding.UTF8, "text/plain"),
        StatusCode = HttpStatusCode.Unauthorized
    };
    return response;
}

I am unable to use the System.Web.Http namespace because of the framework restriction.