IsAjaxRequest

具体来说，IsAjaxRequest代码可以分解为以下功能：

public static bool IsAjaxRequest(this HttpRequestBase request)
{
    if (request == null)
    {
        throw new ArgumentNullException("request");
    }

    return (request["X-Requested-With"] == "XMLHttpRequest") || ((request.Headers != null) && (request.Headers["X-Requested-With"] == "XMLHttpRequest"));
}