Current Async TODOs

@andrewgodwin

It looks like the StaticFilesHandlerMixin is missing the the async response function.

Without this, this is the traceback

Exception inside application: 'NoneType' object is not callable
Traceback (most recent call last):
  File ".../lib/python3.7/site-packages/daphne/cli.py", line 30, in asgi
    await self.app(scope, receive, send)
  File ".../src/django/django/contrib/staticfiles/handlers.py", line 86, in __call__
    return await super().__call__(scope, receive, send)
  File ".../src/django/django/core/handlers/asgi.py", line 161, in __call__
    response = await self.get_response_async(request)
  File ".../src/django/django/core/handlers/base.py", line 148, in get_response_async
    response = await self._middleware_chain(request)
TypeError: 'NoneType' object is not callable

I forgot to make it a pr to ask for a review on github. I’ll try and remember do that next time. Let me know if that seems right to you and it was just missing.