code: func SetHttpClient(httpClient *http.Client) ClientOptionFunc {
  	return func(c *Client) error {
  		if httpClient != nil {
  			c.c = httpClient
  		} else {
  			c.c = http.DefaultClient
  		}
  		return nil
  	}
  }
docs: // SetHttpClient can be used to specify the http.Client to use when making
  // HTTP requests to Elasticsearch.