Use surrogateescape to encode headers on websockets implementation

[sephioh]

Currently, any non-ascii header value sent to uvicorn running with websockets implementation will raise a UnicodeEncodeError.
As websockets package represents non-ascii characters in headers using surrogate escapes, this PR makes websockets implementation to follow the same pattern when encoding them.

Edit by @Kludex:

• Closes WebSocket headers encoding problem #968

[euri10]

closing this until we agree on a fix, let's discuss that in #968

[Kludex]

This matches the behavior with wsproto.

You can confirm it with the following client code:

#!/usr/bin/env python

import asyncio

import websockets


async def hello():
    async with websockets.connect(
        "ws://localhost:8000", extra_headers={"haha": "Abraão"}
    ) as websocket:
        await websocket.send("Hello world!")
        await websocket.recv()


asyncio.run(hello())

And on the uvicorn side, you can run the following using wsproto:

uvicorn main:app --port 8000 --reload --ws wsproto

And... Replace wsproto by websockets above to check the matching behavior.

@tomchristie can you please confirm that we are fine with this?

[tomchristie]

I don't have a problem with this.