Add support for functools.partial in WebsocketRoute

[aminalaee]

Fixes #1355.

import functools

from starlette.applications import Starlette
from starlette.routing import WebSocketRoute

async def handle_ws(websocket):
    pass

app = Starlette(debug=True, routes=[
    WebSocketRoute("/", functools.partial(handle_ws)),
])

Support for using functools.partial with Route is already added. This PR adds support for WebSocketRoute.

[aminalaee]

I'll try to refactor the tests for partial Route tests.

[Kludex]

LGTM!

Thanks @aminalaee ! 🎉