Requests with CancelToken return 403 if host requires valid User-Agent

[Sockitos]

New Issue Checklist

• [x] I have searched for similar issues in the project and found the following:
• #939
• #1166
• #1167

But they are all closed and lack relevant information.

Issue Info

Info	Value
Platform Name	Flutter
Platform Version	3.0.1
Dio Version	4.0.6
Android Studio / Xcode Version	Android Studio 2020.3
Repro rate	all the time (100%)
Repro with our demo prj	-
Demo project link	-

Issue Description and Steps

Every request made with a CancelToken to specific hosts that require a valid User-Agent, return a 403 forbidden response. If you use a baseUrl of a host that requires a valid User-Agent and do the following request:

Future<void> testRequest(
  String url,
  String path,
  CancelToken cancelToken,
) async {
  final dio = Dio(BaseOptions(baseUrl: url));
   final response = await dio.get<Map<String, dynamic>>(
    path,
    cancelToken: cancelToken, 
  );
}

You would get a 403 forbidden, but if you remove the CancelToken, the request would be successful.

It is my understanding that this only happens due to a change made in 516825fb8392e5a8aaaef3cbba6f7cb8d075a620 https://github.com/flutterchina/dio/blob/516825fb8392e5a8aaaef3cbba6f7cb8d075a620/dio/lib/src/adapters/io_adapter.dart#L104

For now I'm just forcing a non-null User-Agent on my request headers, but I still consider this an issue because Dio only enforces a null User-Agent when the request has a CancelToken.

If the line of code that I linked above is really required, I would suggest to always force a null User-Agent (with or without CancelToken) or alert the users through the package documentation.

Thank you!

[wilkier]

Same problem here, @Sockitos temporary solution saved me, thanks.