public void Filter_for_period()
        {
            var formHelper = new FormHelper();

            PrepareHelper(formHelper);
            context.CurrentControllerContext.Helpers.Add(formHelper);
            context.Services.ViewEngineManager = ForTest.GetViewManager();

            filter.Period = new DatePeriod();
            var result = helper.FilterFor("filter.Period");

            Assert.That(result, Is.StringContaining("calendar"));
        }