[BUG][dart-dio] Accept header not generated

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

dart-dio generator seems to not generate Accept header even though it is specified in responses.<code>.content.<media-type>. According to the docs I would assume that the generator sets the Accept header to <media-type>. This seems not to happen. Am I missing something here?

openapi-generator version

6.5.0

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: OpenAPI definition
  version: v0
paths:
  /test:
    get:
      responses:
        "200":
          description: Test
          content:
            application/json:
              schema:
                type: string
                format: uuid

Suggest a fix

The specs above should generate something like this:

Future<Response<String>> testGet({
  CancelToken? cancelToken,
  Map<String, dynamic>? headers,
  Map<String, dynamic>? extra,
  ValidateStatus? validateStatus,
  ProgressCallback? onSendProgress,
  ProgressCallback? onReceiveProgress,
}) async {
  final _path = r'/test';
  final _options = Options(
    method: r'GET',
    headers: <String, dynamic>{
      'Accept': 'application/json',          // <-- this header should be generated
      ...?headers,
    },
    extra: <String, dynamic>{
      'secure': <Map<String, String>>[],
      ...?extra,
    },
    validateStatus: validateStatus,
  );

  // ...
}

Start by generating the dart-dio client from the minimal OpenAPI YAML in the report and inspect the request options for GET /test. Trace how the dart-dio generator handles response content types, then verify that application/json produces an Accept header in the generated method.