How do I test an ActionFilterAttribute with ASP.NET Web API?

I'm building an ASP.NET Web API that uses a very simple action filter:

public class ValidateModelAttribute : ActionFilterAttribute {
    public override void OnActionExecuting(HttpActionContext actionContext) {
        if (actionContext.ModelState.IsValid == false) {
            var responseObject = new ApiResponse() {
                Errors = actionContext.ModelState.Values.SelectMany(v => v.Errors).Select(e => e.ErrorMessage)
            };

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

My action filter simply enforces that the ModelState is valid; if not, it returns a 400 with all the error messages so the client can do whatever they wish with the data.

Here is how I'm using the attribute in my controller:

[ValidateModelAttribute]
public HttpResponseMessage Db(DbModel model) {
    return Request.CreateResponse(HttpStatusCode.OK);
}

I'm trying to test the attribute using Visual Studio's testing framework:

[TestMethod]
public void CaptureApiDatabase_IfRequiredFieldMissing_ReturnHttpBadRequest() {
    var controller = new ConfigureController();
    controller.Request = new HttpRequestMessage();
    controller.Configuration = new HttpConfiguration();            

    var validationAttribute = new ValidateModelAttribute();
    validationAttribute.OnActionExecuting(controller.ActionContext);
    Assert.IsTrue(controller.ActionContext.Response.StatusCode == HttpStatusCode.BadRequest);
}

This isn't working because I haven't provided the filter my data and as a result the ModelState is always valid. How do I build a context so I can test this filter attribute?