pass-through for Schema-based results

[andreasnuesslein]

When the response-parameter is filled and the result is already a Schema we don't need to verify it again.

Brief reasoning for the situation:
Wagtail has a [new guide how to integrate Ninja](based on https://docs.wagtail.org/en/v7.0/advanced_topics/api/django-ninja.html).
In it they use this

@api.get("/pages/{page_id}/", response=BlogPageSchema | HomePageSchema)
def get_page(request: "HttpRequest", page_id: int):
    return get_object_or_404(Page, id=page_id).specific

This works but has its issues since basically the returned Page type (the django Model) is tried to be fitted against all different response-Schemata. Obviously that's a performance issue but it brings other problems too, which I dont want to go too deep into.

So my solution would be:

@api.get("/pages/{page_id}/", response=BlogPageSchema | HomePageSchema)
def get_page(request: "HttpRequest", page_id: int):
    page = get_object_or_404(Page, id=page_id).specific
    if isinstance(page,BlogPage):
        return BlogPageSchema.from_orm(page,context={"request":request})
    return HomePageSchema.from_orm(page,context={"request":request})

this does currently not work, because the result is being re-parsed, because I have response set in the decorator. Obviously I could remove that but then I'd lose the types.

This PR allows Schemas to be directly returned, assuming that the validation already has happened.

Thanks so much