[BUG] Headers not copied into Raw response headers array.

Bug Report Checklist

• [ y] Have you provided a full/minimal spec to reproduce the issue?
• [ y] Have you validated the input using an OpenAPI validator (example)?
• [ y] What's the version of OpenAPI Generator used?
• [ y] Have you search for related issues/PRs?
• [ y] What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

When calling this method I know from the documents that the response provides details of a document created in the header.location but I can't examine this with the typescript-fetch API? The following snippet from a React test app is where I see the response headers are empty. If I use the standard dap

var config = new Configuration({
    apiKey: "ApiKey mykeywashere:mysecret",
    basePath: 'https://<basepath>.amazonaws.com/v1_cors'
  });

  dapi = new DocumentApi(config);

  var d = new DocumentPostFromJSON({
    "group": "/api/v1/group/legalesigndev/",
    "name": "SDK Special Test Document",
    "text": "<h1>Automatic Unit Test Document</h1><p>terms as follows.. <span class=\"field\" data-optional=\"false\" data-name=\"Please add your ...\" data-idx=\"0\" data-signee=\"1\" data-type=\"signature\" data-options=\"\"> ....... </span></p>",
    "signers": [{ "firstname": "fname", "lastname": "lname", "email": "lex@test.com", "order": 0 }],
    "do_email": "false",
    "offset": "",
    "archived": "false",
    "limit": "",
    "filter": ""
  });
  var dpr = {
    "documentPost": d
  };
  console.log("Testing postDocument");
  const out = await dapi.postDocumentRaw(dpr);
  console.log(await out);
  console.log(await out.raw.headers)

openapi-generator version

4.3.1

OpenAPI declaration file content or url

https://raw.githubusercontent.com/legalesign/Legalesign-V1-OpenAPI3/master/legalesign-api-v1.yaml

Command line used for generation

openapi-generator generate -i https://raw.githubusercontent.com/legalesign/Legalesign-V1-OpenAPI3/master/legalesign-api-v1.yaml -g typescript-fetch --additional-properties supportsES6=true

Steps to reproduce

Execute generation, make any Raw Api call - check response.headers.

Related issues/PRs

https://github.com/swagger-api/swagger-codegen/issues/7828

Suggest a fix

Should the baseApi.fetchApi.response be mapping onto this?

Start with the typescript-fetch generator output and the baseApi.fetchApi.response path mentioned in the report. Reproduce the issue using the supplied OpenAPI declaration, generation command, and a Raw API call, then verify that response.headers contains the server headers, including header.location.