Hi, Not sure if this a bug, or an intended feature. 
I got surprise by the following behavior. 


    from asyncio import run, sleep, get_event_loop

    print(get_event_loop()) # return the current eventloop
    run(sleep(0))
    print(get_event_loop()) # raise a RuntimeError


I would expect `get_event_loop` to get back to it's initial default value. 

This comes from the fact that `run()` call `set_event_loop()` to `None`. with
both sets `_set_called` to `True`, but `get_event_loop` seem to assume if
_set_called is True, then loop cannot be none. 

I'm tempted to think that if `set_loop()` is called with `None`, it should reset the `_set_called` to False. Or Am I supposed to call `set_event_loop(new_event_loop())` myself ? 

I'm likely missing  something so any insight would be appreciated; if you
believe this is an actual issue I'm happy to send a PR.