iOS修改webview的UserAgent資訊.

在啟動時，比如在AppDelegate 中呼叫如下程式碼：

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])
}