更改 uiwebview 的useragent

UIWebView* webView = [[UIWebView alloc] initWithFrame:CGRectZero];  
NSString* secretAgent = [webView stringByEvaluatingJavaScriptFromString:@"navigator.userAgent"];

NSString *newUagent = [NSString stringWithFormat:@"%@ appname/3.5.2",secretAgent];  
NSDictionary *dictionary = [[NSDictionary alloc]  
                                initWithObjectsAndKeys:newUagent, @"UserAgent", nil nil];  
[[NSUserDefaults standardUserDefaults] registerDefaults:dictionary];