OC修改：
NSString*userAgent = [[[UIWebViewalloc]init] stringByEvaluatingJavaScriptFromString:@"navigator.userAgent"];
NSString*newUagent = [NSStringstringWithFormat:@"%@ app/iOS",userAgent];
NSDictionary*dictionary = @{@"UserAgent":newUagent}; [[NSUserDefaultsstandardUserDefaults] registerDefaults:dictionary];
swift修改：
let oldAgent = UIWebView().stringByEvaluatingJavaScript(from: "navigator.userAgent")
let newAgent = oldAgent! +"app/iOS"
UserDefaults.standard.register(defaults: ["UserAgent": newAgent])
修改方法需放在Appdelegate的 didFinishLaunchingWithOptions 方法中实现才有效果
