[BUG][csharp-netcore] HttpClient always uses "application/json" as Content-Type even other type is specified.

This is a bit of a poor bug report, I'll fill it out later if required, and if this change is acceptable I'll make a quick PR. Since it will be a mayor behaviour change to fix it.

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

We have some Patch endpoint which only accept application/json-patch+json Content-Type.

These lines: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/csharp-netcore/libraries/httpclient/ApiClient.mustache#L379-L383=

Should probably be:

{
    contentType = contentType ?? "application/json";

    var serializer = new CustomJsonCodec(SerializerSettings, configuration);
    request.Content = new StringContent(serializer.Serialize(options.Data), new UTF8Encoding(),
        contentType);
}

openapi-generator version

6.0.0-SNAPSHOT (:latest docker tag)

OpenAPI declaration file content or url

requestBody:
    content:
      application/json-patch+json:
        schema:
          $ref: '#/components/schemas/JsonPatchDocument'

Generation Details

This is for the csharp-netcore generator with the library: httpclient config option. Using the :latest docker tag.

Steps to reproduce

1. Do request to server
2. Check Content-Type that the server receives. (It's application/json instead of application/json-patch+json)

Related issues/PRs

Suggest a fix

These lines: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/csharp-netcore/libraries/httpclient/ApiClient.mustache#L379-L383=

Should probably be:

{
    contentType = contentType ?? "application/json";

    var serializer = new CustomJsonCodec(SerializerSettings, configuration);
    request.Content = new StringContent(serializer.Serialize(options.Data), new UTF8Encoding(),
        contentType);
}

Start with modules/openapi-generator/src/main/resources/csharp-netcore/libraries/httpclient/ApiClient.mustache around lines 379-383 and inspect how the request content type is selected. Generate a csharp-netcore client with the httpclient library for an application/json-patch+json request body, then confirm the generated request preserves that media type instead of always using application/json.