code: @Nonnull
    public Sse keepAlive(final int time, final TimeUnit unit) {
      return keepAlive(unit.toMillis(time));
    }
docs: The keep alive time can be used to prevent connections from timing out:
  
  <pre>{@code
  {
  sse("/events/:id", sse -> {
  sse.keepAlive(15, TimeUnit.SECONDS);
  });
  }
  }</pre>
  
  <p>
  The previous example will sent a <code>':'</code> message (empty comment) every 15 seconds to
  keep the connection alive. If the client drop the connection, then the
  {@link #onClose(Throwing.Runnable)} event will be fired it.
  </p>
  
  <p>
  This feature is useful when you want to detect {@link #onClose(Throwing.Runnable)} events without
  waiting until you send a new event. But for example, if your application already generate
  events
  every 15s, then the use of keep alive is useless and you should avoid it.
  </p>
  
  @param time Keep alive time.
  @param unit Time unit.
  @return This instance.