code: func (c *Client) Unsubscribe(topic string) (GenericFuture, error) {
  	return c.UnsubscribeMultiple([]string{topic})
  }
docs: // Unsubscribe will send a Unsubscribe packet containing one topic to unsubscribe.
  // It will return a UnsubscribeFuture that gets completed once an Unsuback packet
  // has been received.