public HttpRequestMessage CreatePost(string url, SortedDictionary<string, string> parameters)
{
var oauth = BuildAuthenticatedResult(url, parameters, "POST");
var fullUrl = url;
var request = new HttpRequestMessage(HttpMethod.Post, fullUrl);
request.Headers.Add("Authorization", oauth.Header);
request.Headers.Add("User-Agent", TwitterApi.UserAgent());
var content = parameters.Aggregate(string.Empty, (current, e) => current + string.Format("{0}={1}&", e.Key, Uri.EscapeDataString(e.Value)));
request.Content = new StringContent(content, Encoding.UTF8, "application/x-www-form-urlencoded");
return request;
}