403 Error when aiohttp resources are not fully merged

Aiohttp merges resources, when they are added in the correct order:

When using aiohttp-cors, resources have to be fully merged, or you end up having 2 resources for the same path, one for the GET method one for the POST, but the latter will not be considered when aiohttp-cors answers the OPTIONS request. It seems that aiohttp-cors supports only a single resource per path.

It would be nice if a hint about this could be added to the docs, if the support of multiple resources for the same path is too complex.

Requirements:
```
aiohttp==3.0.6
aiohttp_cors==0.7.0
```

Replicate:
```
def handler():
pass

app.router.add_route('POST', '/a', handler)
app.router.add_route('GET', '/b', handler)
app.router.add_route('PUT', '/a', handler)

# Configure default CORS settings.
cors = aiohttp_cors.setup(app, defaults={
"*": aiohttp_cors.ResourceOptions(
allow_credentials=True,
expose_headers="*",
allow_headers="*")
})

# Configure CORS on all routes.
for route in list(app.router.routes()):
cors.add(route)

```