text: Title: [BUG] duplicated mapping key on adding response header types for openapi/swagger 
  Body: I wanted set returned reponse header types for my `login/` API with `openapi_extra` param, but it is not allowing me to do so.
  ```python
  @router.get("login/", response={200: AuthOutSchema, 400: MessageOut}, auth=None, openapi_extra={
      "responses": {
          "200": {
              "headers": {
                  "Set-Cookie": {
                      "type": "string",
                      "description": "Session Id Token",
                  }
              },
              "description": "successful operation",
          }
      }
  })
  @csrf_exempt
  def login_user(request, username: str, password: str):
  ```
  
  If I change it to `203` the response allows me to use the response header feature.
  
  **Versions (please complete the following information):**
   - Python version: 3.11.2
   - Django version: 4.1.6
   - Django-Ninja version: 0.22.2
   - Pydantic version: 1.10.13
  
  