// Update method changes configuration of an existing heartbeat at OpsGenie.
func (cli *OpsGenieHeartbeatClient) Update(req heartbeat.UpdateHeartbeatRequest) (*heartbeat.UpdateHeartbeatResponse, error) {
	req.APIKey = cli.apiKey
	resp, err := cli.sendRequest(cli.buildPostRequest(updateHeartbeatURL, req))

	if resp == nil {
		return nil, err
	}
	defer resp.Body.Close()

	var updateHeartbeatResp heartbeat.UpdateHeartbeatResponse
	if err = resp.Body.FromJsonTo(&updateHeartbeatResp); err != nil {
		message := "Server response can not be parsed, " + err.Error()
		logging.Logger().Warn(message)
		return nil, errors.New(message)
	}

	return &updateHeartbeatResp, nil
}