fix _run_authentication creating the coroutine result twice

[eugenenelou]

This fix a problem where _run_authentication in sync mode and async callback instantiate the coroutine twice and awaits the second one.

This raises a "coroutine not awaited" warning, which is annoying.

[matez0]

In the try block,

                result = callback(request)
                if inspect.iscoroutine(result):
                    async def await_result(cor: Coroutine) -> Any:
                        return await cor

                    result = async_to_sync(await_result)(result)

would not be enough since result is created the same way in all cases and we may want to await result independently of the properties of callback?

[vitalik]

@eugenenelou could you show example code(authenticator?) that triggers "coroutine not awaited" warning ?

Ideally include it tests