"return json(true,jsonrequestbehavior.allowget)" returns 'true' on screen instead of calling onsuccess method

<appsettings>
    <add key="ClientValidationEnabled" value="true" />
    <add key="UnobtrusiveJavaScriptEnabled" value="true" />
</appsettings>

@using (Ajax.BeginForm("PerformAction", new AjaxOptions { OnSuccess = "OnSuccess", HttpMethod="POST", OnFailure = "OnFailure" }))
{
    <fieldset>
        <input type="submit" value="Submit" />
    </fieldset>
}

<script src="~/Scripts/jquery.unobtrusive-ajax.min.js"></script>
<script type="text/javascript">

    function OnSuccess(response) {
        alert(response);
    }

    function OnFailure(response) {
        alert("Something wrong happens. Error is: " + response);
    }

</script>

[HttpPost]
public JsonResult performAction()
{
	return Json(true, JsonRequestBehavior.AllowGet);
}