title: Unit testing ModelState ActionFilter
body: <p>guys! 
  I have this ModelState filter, </p>
  
  <pre><code>public class ValidateModelStateFilter : ActionFilterAttribute
  {
      public override void OnActionExecuting(HttpActionContext actionContext)
      {
          if (!actionContext.ModelState.IsValid)
          {
              actionContext.Response = actionContext.Request.CreateErrorResponse(
                  HttpStatusCode.BadRequest, actionContext.ModelState);
          }
      }
  }
  </code></pre>
  
  <p>But I don't have any ideas on how to unit test it.
  Any help?</p>