[BUG] [kotlin] Wrong implicit Accept header sent when making requests to endpoint that expects HTTP 204

Bug Report Checklist

Description

When not specifying explicit header parameters the generated client will automatically add an Accept header application/json to all requests. This works nice in most cases but not when an endpoint returns 204, No content.

A request with Accept header application/json then results in HTTP 406 from the server

openapi-generator version

7.4.0

OpenAPI declaration file content or url

  /test/property/{property_id}:
    patch:
      tags:
        - 'property'
      summary: 'Update property details'
      description: 'Modify ''managedBy'' property value'
      operationId: 'updateProperty'
      parameters:
        - $ref: '#/components/parameters/PropertyId'
      requestBody:
        content:
          'application/vnd.xyz-0.1+json':
            schema:
              $ref: '#/components/schemas/PropertyUpdate'

      responses:
        '204':
          description: 'Successful operation'
        '400':
          $ref: '#/components/responses/400'
        '401':
          $ref: '#/components/responses/401'
        '404':
          $ref: '#/components/responses/404'
        '406':
          $ref: '#/components/responses/406'
        '415':
          $ref: '#/components/responses/415'
        '500':
          $ref: '#/components/responses/500'
        '503':
          $ref: '#/components/responses/503'
        default:
          $ref: '#/components/responses/GeneralError'

Generation Details

openApiGenerate {
    generatorName.set("kotlin")
    // Some irrelevant config details are omitted
    with (configOptions) {
        put("dateLibrary", "java17")
        put("serializationLibrary", "kotlinx_serialization")
    }
}

Suggest a fix

Whenever 204 is expected, provide the Accept header set to */* instead of application/json

Start by reproducing the Kotlin client generated from the shown PATCH operation with the kotlinx_serialization library, then trace how request generation selects the Accept header for a 204 response. Verify the generated request behavior against the provided OpenAPI declaration, including the expected handling of explicit headers and the 204 response.