Bug Report Checklist

• [~] Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator, yes I used the validate function of the api generator
• [5.0.0 ] What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Expected behaviour

The following code snippet:

    RestClient::Connection* conn = new RestClient::Connection("https://httpbin.org");
    RestClient::Response& r = conn->get("/get");
    for(auto& name : { "date", "Date" }) {
        try {
            std::string& value = r.headers.at(name);
            std::cout << name << ": \"" << value << "\"" << std::endl;
        } catch(st

It would be great to handle subgroups/subfolders as the following image: