add jsonp support for .net web api

JsonpAttribute.cs

public class JsonpAttribute : ActionFilterAttribute
{
  private const string CallbackQueryParameter = "callback";
  
  public override void OnActionExecuted(HttpActionExecutedContext context)
  {
    var callback = string.Empty;

    if (IsJsonp(out callback))
    {
      var jsonBuilder = new StringBuilder(callback);

      jsonBuilder.AppendFormat("({0})", context.Response.Content.ReadAsStringAsync().Result);

      context.Response.Content = new StringContent(jsonBuilder.ToString());
    }

    base.OnActionExecuted(context);
  }
  
  private bool IsJsonp(out string callback)
  {
    callback = HttpContext.Current.Request.QueryString[CallbackQueryParameter];

    return !string.IsNullOrEmpty(callback);
  }
}