Add headers parameter for HTTP::WebSocket constructors

[adamtrilling]

Added a headers parameter to both constructors for HTTP::WebSocket, to be used as follows:

HTTP::WebSocket.new(URI.parse("ws://websocket.example.com/chat"),
  HTTP::Headers{"Authorization" => "Bearer authtoken"})

Now you can pass headers along with your websocket request, for authorization and whatnot. I've created an issue for this (#4222) but it was so easy to implement that I just went ahead and did it.

[bcardiff]

So, this PR allows to send headers while creating a websocket when using crystal as client.

It does not cover accessing headers in the websocket in http server, since the headers are not accessible from a HTTP::WebSocket AFAIK and there is no change in the line where a websocket is created for the server side counter part: https://github.com/crystal-lang/crystal/blob/master/src/http/server/handlers/websocket_handler.cr#L33

Although I see value in supporting in a nicer way both headers and querystring, I think it's support needs to arrive in both sides of the http protocol at the same side.

[asterite]

I would merge this right now. This lets you open a websocket connection with headers specified, regardless of whether you are going to provide a websocket server, so it's useful by itself.

[RX14]

soo, can we merge this?

[mverzilli]

Thanks @adamtrilling !

[RX14]

@bcardiff Actually, doesn't WebSocketHandler take a block with arguments for both the websocket and the HTTP::Server::Context, which would allow you to access the request headers.