[BUG] [Fetch] generate text response instead of json response

Created by: andreafalco1991

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue? yes
• Have you validated the input using an OpenAPI validator (example)? yes
• What's the version of OpenAPI Generator used? 4.0.0
• Have you search for related issues/PRs? yes

Description

openapi-generator version

4.0.0

OpenAPI declaration file content or url

swagger: "2.0"
info:
  version: "0"
  title: test
  
host: localhost
schemes:
  - https
paths:
  /get-all-elements:
    get:
      tags:
        - linea
      operationId: getAllElements
      produces:
        - application/json
      responses:
        "200":
          description: test
          schema:
            type: object

Command line used for generation

openapi-generator generate -i ./api.yaml -g typescript-fetch -o ./src/types/generated

Steps to reproduce

just generating the code

Suggest a fix

If the type of the schema of response is object it should generate a json response.