ASP.NET MVC:can not add custom header in Ajax Request of Bootstrap modal dialog

Question

I am trying to add custom header in ajax but failed in bootstrap modal dialog,if I put below code in the normal page but the popup page ,it can work fine. the code looks like these:

     var token = $('@Html.AntiForgeryToken()').val();
        var headers = {};
        headers["__RequestVerificationToken"] = token;
        console.log(token);
        $.ajax({
            type: 'POST',
            url: '@Url.Action("SignGainPoint")',
            cache: false,
            data: null,
            //beforeSend: function (request) {
            //    request.setRequestHeader("__RequestVerificationToken", token);
            //},
            headers:headers,
            success: function (e) {
                if (e.IsSuccess) {
                    $(".today").children("span").html(e.data);
                }
                else {
                    $(".today").children("span").html(0);
                }
            },
            error: function () {
                alert("Error");
            }
        });
    })

and background code,the request.Headers["__RequestVerificationToken"] is null in asp.net mvc code.

public class MyValidateAntiForgeryToken : AuthorizeAttribute
{
    public override void OnAuthorization(AuthorizationContext filterContext)
    {
        var request = filterContext.HttpContext.Request;
        if (request.HttpMethod== WebRequestMethods.Http.Post)
        {
            if (request.IsAjaxRequest())
            {
                var antiForgeryCookie = request.Cookies[AntiForgeryConfig.CookieName];
                var cookieValue = antiForgeryCookie != null
                    ? antiForgeryCookie.Value : null;
                AntiForgery.Validate(cookieValue, request.Headers["__RequestVerificationToken"]);
            }
            else
            {
                new ValidateAntiForgeryTokenAttribute()
  .OnAuthorization(filterContext);
            }
        }
    }

}