code: @Nonnull
    public CompletableFuture<Optional<Object>> send(final Object data, final String type) {
      return send(data, MediaType.valueOf(type));
    }
docs: Send an event and set media type.
  
  <pre>{@code
  sse.send(new MyObject(), "json");
  }</pre>
  
  <pre>{@code
  sse.send(new MyObject(), "json").whenComplete((id, x) -> {
  if (x == null) {
  handleSuccess();
  } else {
  handleError(x);
  }
  });
  }</pre>
  
  The <code>id</code> of the success callback correspond to the {@link Event#id()}.
  
  @param data Event data.
  @param type Media type, like: json, xml.
  @return A future. The success callback contains the {@link Event#id()}.