Daphne ignores "headers" section from "websocket.accept" message

I want to add `permessage_deflate` Websocket extension support for `django_channels` consumer/server. Server has to provide supported extensions in the headers of handshake response, e.g. `Sec-Websocket-Extensions: permessage-deflate`. [ASGIREF documentation](https://asgi.readthedocs.io/en/latest/specs/www.html#accept-send-event) states, that `"headers"` section is supported for `"websocket.accept"` message type. Hence, I have tried to accept Websocket connection with this code:

```
from channels.generic.websocket import AsyncJsonWebsocketConsumer

class CeleryTaskConsumer(AsyncJsonWebsocketConsumer):
async def connect(self):
await self.base_send({
"type": "websocket.accept",
"headers": [
(b"sec-websocket-extensions", b"permessage-deflate")
]
})
```
However, the daphne server ignores the `"headers"` section of this message and only uses `"subprotocol"` section:
https://github.com/django/daphne/blob/master/daphne/ws_protocol.py#L185

That means, that Daphne server has no way to inform client, that it supports some kind of Websocket extensions, which is a major limitation.

pip freeze:
```
asgiref==3.2.10
attrs==19.3.0
autobahn==20.7.1
Automat==20.2.0
cffi==1.14.1
channels==2.4.0
constantly==15.1.0
cryptography==3.0
daphne==2.5.0
Django==2.2
hyperlink==19.0.0
idna==2.10
incremental==17.5.0
pyasn1==0.4.8
pyasn1-modules==0.2.8
pycparser==2.20
PyHamcrest==2.0.2
pyOpenSSL==19.1.0
pytz==2020.1
service-identity==18.1.0
six==1.15.0
sqlparse==0.3.1
Twisted==20.3.0
txaio==20.4.1
zope.interface==5.1.0
```