C# HttpWebRequest Content-type not Changing

In C# i need to POST some data to a web server using HTTP. I keep getting errors returned by the web server and after sniffing throught the data I dound that the problem is that thew Content-type header is still set to "text/html" and isn't getting changed to "application/json; Charset=UTF-8" as in my program. I've tried everything I can think of that might stop it getting changed, but am out of ideas.

Here is the function that is causing problems:

private string post(string uri, Dictionary<string, dynamic> parameters)
    {
        //Put parameters into long JSON string
        string data = "{";
        foreach (KeyValuePair<string, dynamic> item in parameters)
        {
            if (item.Value.GetType() == typeof(string))
            {
                data += "\r\n" + item.Key + ": " + "\"" + item.Value + "\"" + ",";
            }
            else if (item.Value.GetType() == typeof(int))
            {
                data += "\r\n" + item.Key + ": " + item.Value + ",";
            }
        }
        data = data.TrimEnd(',');
        data += "\r\n}";

        //Setup web request
        HttpWebRequest wr = (HttpWebRequest)WebRequest.Create(Url + uri);
        wr.KeepAlive = true;
        wr.ContentType = "application/json; charset=UTF-8";
        wr.Method = "POST";
        wr.ContentLength = data.Length;
        //Ignore false certificates for testing/sniffing
        wr.ServerCertificateValidationCallback = delegate { return true; };
        try
        {
            using (Stream dataStream = wr.GetRequestStream())
            {
                //Send request to server
                dataStream.Write(Encoding.UTF8.GetBytes(data), 0, data.Length);
            }
            //Get response from server
            WebResponse response = wr.GetResponse();
            response.Close();
        }
        catch (WebException e)
        {
            MessageBox.Show(e.Message);
        }
        return "";
    }

The reason i'm getting problems is because the content-type stays as "text/html" regardless of what I set it as.

Thanks in advence.