How can i execute http request from not-main thread in unity?

Question

I use WWW library for Unity in C#. When i call function Send from main thread it's ok.

PostWorker pw = new PostWorker();    
API.instance.StartCoroutine(pw.Send(url, postParams));

But when i try make it from PostWorker

API.instance.StartCoroutine(Send(url, postParams));

it's not work with such error:

StartCoroutine_Auto can only be called from the main thread.

If I call the function without Coroutine it doesn't work and not enter in Send.

private IEnumerator Send(string path, Dictionary<string, string> postData)
{
    WWWForm form = new WWWForm();
    if (postData != null && postData.Any())
    {
        foreach (var item in postData)
        {
            form.AddField(item.Key, item.Value);
        }
    }

    WWW www = new WWW(path, form);
    yield return www;

    if (string.IsNullOrEmpty(www.error))
    {
        Debug.Log(www.text);
    }
    else
    {
        Debug.Log(www.error);
    }
}

So, how can i execute http request from not-main thread?