Authentication middleware prevent CORS to be send correctly

I have an authentication middleware. In the middleware if the request method is OPTIONS I am returning the handler intact with the aim that aiohttp-cors handle the preflight request and return the correct response headers. However, the response headers are not being sent correctly by the signals.

It is quite possible that I am doing something wrong in my middleware, and the OPTIONS call need to be handled differently. This is my middleware:

```python
@middleware
async def auth_middleware(request, handler):
if isinstance(request.match_info.route, SystemRoute): # eg. 404
return await handler(request)

if request.method == hdrs.METH_OPTIONS:
return await handler(request)

try:
request['claims'] = await authenticate(request)
except ValueError as e:
raise HTTPUnauthorized(PayloadErrors(e.args[0]))

return await handler(request)
```

I am creating the app as following:
```python
def create_app():
app = Application(middlewares=middlewares)
setup_cors(app)
return app
```

And this is how I am setting up cors:
```python
def setup_cors(app: Application):
resources = [
'http://localhost:8100',
'http://www.example.com',
]

cors = aiohttp_cors.setup(app, defaults={
resource: aiohttp_cors.ResourceOptions(
allow_credentials=True,
expose_headers='*',
allow_methods='*',
allow_headers='*',
) for resource in resources
})

for route in app.router.routes():
cors.add(route)
```

However, whenever I make a call I get the following error:

```python
Unhandled exception
Traceback (most recent call last):
File "/python3.7/site-packages/aiohttp/web_protocol.py", line 398, in start
await resp.prepare(request)
File "/python3.7/site-packages/aiohttp/web_response.py", line 299, in prepare
await request._prepare_hook(self)
File "/python3.7/site-packages/aiohttp/web_request.py", line 686, in _prepare_hook
await app.on_response_prepare.send(self, response)
File "/python3.7/site-packages/aiohttp/signals.py", line 35, in send
await receiver(*args, **kwargs)
File "/python3.7/site-packages/aiohttp_cors/cors_config.py", line 171, in _on_response_prepare
assert hdrs.ACCESS_CONTROL_ALLOW_ORIGIN not in response.headers
AssertionError
Unhandled exception
Traceback (most recent call last):
File "/python3.7/site-packages/aiohttp/web_protocol.py", line 398, in start
await resp.prepare(request)
File "/python3.7/site-packages/aiohttp/web_response.py", line 299, in prepare
await request._prepare_hook(self)
File "/python3.7/site-packages/aiohttp/web_request.py", line 686, in _prepare_hook
await app.on_response_prepare.send(self, response)
File "/python3.7/site-packages/aiohttp/signals.py", line 35, in send
await receiver(*args, **kwargs)
File "/python3.7/site-packages/aiohttp_cors/cors_config.py", line 171, in _on_response_prepare
assert hdrs.ACCESS_CONTROL_ALLOW_ORIGIN not in response.headers
AssertionError
```