[BUG] [cpprestsdk] multipart/related not correctly handled

Bug Report Checklist

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

Description

The generator produces the code with the expected http content type but the corresponding code has not been generated, so that an exception is raised.

Generated code looks like :

pplx::task<utility::string_t> SMContextsCollectionApi::postSmContexts(std::shared_ptr<Inline_object> inlineObject)
{
 ../..
    std::unordered_set<utility::string_t> localVarConsumeHttpContentTypes;
    localVarConsumeHttpContentTypes.insert( utility::conversions::to_string_t("multipart/related") );

    std::shared_ptr<IHttpBody> localVarHttpBody;
    utility::string_t localVarRequestHttpContentType;

    // use JSON if possible
    if ( localVarConsumeHttpContentTypes.size() == 0 || localVarConsumeHttpContentTypes.find(utility::conversions::to_string_t("application/json")) != localVarConsumeHttpContentTypes.end() )
    {
        localVarRequestHttpContentType = utility::conversions::to_string_t("application/json");
        web::json::value localVarJson;

        localVarJson = ModelBase::toJson(inlineObject);
        localVarHttpBody = std::shared_ptr<IHttpBody>( new JsonBody( localVarJson ) );
    }
    // multipart formdata
    else if( localVarConsumeHttpContentTypes.find(utility::conversions::to_string_t("multipart/form-data")) != localVarConsumeHttpContentTypes.end() )
    {
        localVarRequestHttpContentType = utility::conversions::to_string_t("multipart/form-data");
        std::shared_ptr<MultipartFormData> localVarMultipart(new MultipartFormData);

        if(inlineObject.get())
        {
            inlineObject->toMultipart(localVarMultipart, utility::conversions::to_string_t("inlineObject"));
        }

        localVarHttpBody = localVarMultipart;
        localVarRequestHttpContentType += utility::conversions::to_string_t("; boundary=") + localVarMultipart->getBoundary();
    }
    else
    {
        throw ApiException(415, utility::conversions::to_string_t("SMContextsCollectionApi->postSmContexts does not consume any supported media type"));
    }
../..
}

"multipart/related" is set correctly but just behind its set, there is no corresponding code so that the ApiException (415) is raised

openapi-generator version

4.0.0-SNAPSHOT - hash 4b414d81d49ae21447d944cca1693a11bb0140dd

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  version: 1.0.0
  title: Check generation of cpprest
  description: Internal ref filename is check_multipart-related.yaml

servers:
  - url: http://localhost:8080

paths:
  /sm-contexts:
    post:
      summary:  Create SM Context
      tags:
        - SM contexts collection
      operationId: PostSmContexts
      requestBody:
        description: representation of the SM context to be created in the SMF
        required: true
        content:
          multipart/related:
            schema:
              type: object
              properties: # Request parts
                dataMessage:
                  type: string

      responses:
        '200':
          description: Everythings gonna be alright
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Content"
        default:
          description: unexpected error


components:
  schemas:
    Content:
      type: string

Command line used for generation

openapi-generator-cli.sh generate -i ./openapi.yaml -g cpp-restsdk -o gen-cpp

Steps to reproduce

Just generate

Related issues/PRs

N/A

Suggest a fix

Generate the corresponding code (>_<) 😂

Reproduce the issue with the provided OpenAPI declaration, the check_multipart-related.yaml case, and the cpp-restsdk command from openapi-generator-cli.sh. Trace request-body handling in the cpp-restsdk generator and compare multipart/related with multipart/form-data. Done means generated code handles multipart/related without raising ApiException 415.