在启动时，比如在AppDelegate 中调用如下代码：
func addedUserAgentMsg() {
let webview = UIWebView.init(frame: CGRectZero)
let oldAgent = webview.stringByEvaluatingJavaScriptFromString("navigator.userAgent")
//print("old UA :\n\(oldAgent)")
let newAgent = oldAgent?.stringByAppendingString(" APPBuildName/APPBuildName APPVersion/2.1.0")
let dict = NSDictionary.init(objects: [newAgent!], forKeys: ["UserAgent"])
UserDefault.registerDefaults(dict as! [String : AnyObject])
}