Fix event message type on closing

[lulingar]

Aims to fix #97.

Unit tests pass, and makes the test code provided in the aforementioned issue to run without exceptions:

import asyncio
import logging
import httpx
from httpx_ws import aconnect_ws
from httpx_ws.transport import ASGIWebSocketTransport
from starlette.applications import Starlette
from starlette.routing import WebSocketRoute
from starlette.websockets import WebSocketDisconnect


async def ws_hello(websocket):
    try:
        await websocket.accept()
        await websocket.send_text("Hello World!")
        await websocket.receive_text()
    except WebSocketDisconnect:
        pass
    except Exception as e:
        logging.exception(e)

app = Starlette(
    routes=[
        WebSocketRoute("/ws", ws_hello),
    ],
)


async def main():
    async with httpx.AsyncClient(transport=ASGIWebSocketTransport(app)) as client:
        async with aconnect_ws("http://server/ws", client) as ws:
            message = await ws.receive_text()
            assert message == "Hello World!"

asyncio.run(main())