add impl Sink for WebSocketConnection

[mcgoo]

I couldn't get futures::select!() to work until I added impl Sink for WebSocketConnection. Without it, I could not work out how to send messages on the stream after fusing it.

[yoshuawuyts]

Hi there, thanks for opening this PR. Unfortunately this is a duplicate of an earlier, closed PR: #3. Not implementing Sink has been a deliberate choice in the http-rs ecosystem. We don't think it's likely this trait will ever make it to the stdlib.

If you want to run multiple futures to completion in parallel, check out async-std's Future::{join,race} methods, and Stream::merge.

[mcgoo]

Got it. Thanks for the pointers, Stream::merge looks like the right thing for what I'm trying to do.

(It'll be nice to avoid futures::select!() if possible - it's powerful and useful but it annoys me a little to write core pieces of an app in almost-rust inside a macro.)