respondWebSocket

    pub fn respondWebSocket(request: *Request, options: WebSocketOptions) ExpectContinueError!WebSocket {
        if (request.head.expect != null) return error.HttpExpectationFailed;

        const out = request.server.out;
        const version: http.Version = .@"HTTP/1.1";
        const status: http.Status = .switching_protocols;
        const phrase = options.reason orelse status.phrase() orelse "";

        assert(request.head.version == version);
        assert(request.head.method == .GET);

        var sha1 = std.crypto.hash.Sha1.init(.{});
        sha1.update(options.key);
        sha1.update("258EAFA5-E914-47DA-95CA-C5AB0DC85B11");
        var digest: [std.crypto.hash.Sha1.digest_length]u8 = undefined;
        sha1.final(&digest);
        try out.print("{s} {d} {s}\r\n", .{ @tagName(version), @backingInt(status), phrase });
        try out.writeAll("connection: upgrade\r\nupgrade: websocket\r\nsec-websocket-accept: ");
        const base64_digest = try out.writableArray(28);
        assert(std.base64.standard.Encoder.encode(base64_digest, &digest).len == base64_digest.len);
        try out.writeAll("\r\n");

        for (options.extra_headers) |header| {
            assert(header.name.len != 0);
            var bufs: [4][]const u8 = .{ header.name, ": ", header.value, "\r\n" };
            try out.writeVecAll(&bufs);
        }

        try out.writeAll("\r\n");

        return .{
            .input = request.server.reader.in,
            .output = request.server.out,
            .key = options.key,
        };
    }