OkHttpClient client = new OkHttpClient();
    EventSource.Factory factory = EventSource.Factory.create(client);
    server.enqueue(
      new MockResponse.Builder()
        .body("data: hello\n\n")
        .setHeader("content-type", "text/event-stream")
        .build()
    );
    Request request = new Request.Builder().url(server.url("/")).build();
    CompletableFuture<Void> future = new CompletableFuture<>();