[Dart-dio] Not correct to processing any response other than json.

openapi-generator-cli 6.4.0
dio: ^5.0.0

If spec has "schema=string" API method will throw exception.

"responses": {
          "200": {
            "content": {
              "text/plain": {
                "schema": {
                  "type": "string"
                }
              }
            }
          }
        }

The error is in this line
https://github.com/OpenAPITools/openapi-generator/blob/c81ff5801d0af944a3012a85e4e47d0813484b6f/modules/openapi-generator/src/main/resources/dart/libraries/dio/api.mustache#L108

I think it has to be
final _response = await _dio.request<{{{returnType}}}{{^returnType}}void{{/returnType}}>(

Start in modules/openapi-generator/src/main/resources/dart/libraries/dio/api.mustache at the referenced request construction. Reproduce generation from a spec with a text/plain string response and verify that the generated Dart Dio client uses the declared return type instead of assuming JSON; done means the generated method handles that response without throwing.