Solve method doesn't return id of captcha task.

[Insadem]

If you call client.Solve(req), you recieve token or error.
If you have desire to report captcha task - you have to use callback parameter to receive id of each task.
Simple solution would be to return id from this method too.

My implementation of this solution:

type SolveResult struct {
	ID    string
	Token string
}

func (c *Client) Solve(req Request) (SolveResult, error) {
	if c.Callback != "" {
		_, ok := req.Params["pingback"]
		if !ok {
			// set default pingback
			req.Params["pingback"] = c.Callback
		}
	}

	pingback, hasPingback := req.Params["pingback"]
	if pingback == "" {
		delete(req.Params, "pingback")
		hasPingback = false
	}

	_, ok := req.Params["soft_id"]
	if c.SoftId != 0 && !ok {
		req.Params["soft_id"] = strconv.FormatInt(int64(c.SoftId), 10)
	}

	id, err := c.Send(req)
	if err != nil {
		return SolveResult{}, err
	}

	// don't wait for result if Callback is used
	if hasPingback {
		return SolveResult{
			ID: id,
		}, nil
	}

	timeout := c.DefaultTimeout
	if req.Params["method"] == "userrecaptcha" {
		timeout = c.RecaptchaTimeout
	}

	result, resultErr := c.WaitForResult(id, timeout, c.PollingInterval)
	return SolveResult{ID: id, Token: result}, resultErr
}