HTTP Post Authentication错误

Question

这是我的代码

string URL = "https://api.fastspring.com/accounts";

using (var wb = new WebClient())
{
    var data = new NameValueCollection();
    var contact = new NameValueCollection();
    data["contact"] = "first="+FirstName+"&last="+LastName+"&email="+Email+"&company="+Company;
    data["language"] = Language;
    data["country"] = Country;

    var headers = new WebHeaderCollection();
    headers["Authorization"] = System.Convert.ToBase64String(System.Text.Encoding.GetEncoding("ISO-8859-1").GetBytes("username:password"));
    headers["User-Agent"] = "request";
    //headers["Content-Type"] = "application/json";

    wb.Headers = headers;
    var response = wb.UploadValues(URL, "POST", data);
    return (System.Text.Encoding.UTF8.GetString(response)).Split('"')[3];
}

我的问题是我继续收到401错误说未经授权。首先，我的身份验证语法是否正确？我的整个设置是否正确地向服务器发送HTTP请求？

这里有一些关于fastspring api的有用资源，如果有帮助的话：

http://docs.fastspring.com/integrating-with-fastspring/fastspring-api/accounts#id-/accounts-Createanaccount

http://docs.fastspring.com/integrating-with-fastspring/fastspring-api