title: iOS 12 WKWebview set customUserAgent not work?
body: <p>It seems there is a bug in iOS 12 when set wkwebview customUserAgent. In webView:didFinishNavigation: method I print customUserAgent and compare with webivew.evaluateJavaScript result.It is different.</p>
  
  <pre><code>NSLog(@"user-agent is %@"); //Get a custom user-agent
  [self.webView evaluateJavaScript:@"navigator.userAgent" completionHandler:^(id _Nullable result, NSError * _Nullable error) {
      NSLog(@"user-agent is %@", result); //Get a default  user-agent
  }];
  </code></pre>
  
  <p>Dose anyone see the same problem?</p>