Question about ServerHttpProtocol.closing()

The method doesn't reset `self.transport` to `None` after `self.transport.close()` call.
That means `self.transport.close()` will be called on next `.closing()` call again.

I see nothing wrong with it because asyncio transports correctly process double `.close()` calls but maybe  adding line like `self.transport = None` make code a bit more accurate?
