HttpsURLConnection BAD REQUEST

I'm trying to send a POST request to Plivo (VoIP server) to send an SMS to my cell phone, but I only get BAD REQUEST (Code 400).

The error I received is "error": "invalid json data sent in raw POST"

I can not find the error.

Does anyone know how to solve it?

final AsyncTask < String, Void, Boolean > request = new AsyncTask < String, Void, Boolean > () {

    @Override
    protected Boolean doInBackground(String...strings) {

        Boolean retorno = true;

        HttpsURLConnection request = null;
        try {
            request = (HttpsURLConnection) finalUrl.openConnection();

            System.out.println("Define POST");
            request.setRequestMethod("POST");
            request.setUseCaches(false);
            request.setDoOutput(true);

            System.out.println("Define propriedades");
            request.setRequestProperty("Content-Type", "application/json");
            request.setRequestProperty("Accept", "application/json");
            request.setRequestProperty("Accept-Charset", "UTF-8");
            request.setRequestProperty("charset", "UTF-8");

            System.out.println("Define autenticação");
            String autenticacao = authID + ":" + authToken;
            String basic = "Basic " + new String(Base64.encode(autenticacao.getBytes(), Base64.NO_WRAP));
            request.setRequestProperty("Authorization", basic);

            System.out.println("Define parâmetros");
            JSONObject params = new JSONObject();
            params.put("dst", numeroDestino);
            params.put("src", numeroOrigem);
            params.put("text", body);

            System.out.println("Faz conexão e requisição");
            request.connect();
            OutputStreamWriter postParaEnvio = new OutputStreamWriter(request.getOutputStream());
            postParaEnvio.write(URLEncoder.encode(params.toString(), "UTF-8"));
            postParaEnvio.flush();
            postParaEnvio.close();

            int codigoStatus = request.getResponseCode();

            for (Map.Entry < String, List < String >> header: request.getHeaderFields().entrySet()) {
                System.out.println(header.getKey() + "=" + header.getValue());
            }

            if (codigoStatus == 202) {
                System.out.println("OK");
            } else {
                System.out.println("Falha");
                System.out.println(codigoStatus);

                retorno = false;
            }

        } catch (IOException e) {
            System.out.println("Falha Exception");
            e.printStackTrace();

            retorno = false;
        } catch (JSONException e) {
            e.printStackTrace();

            retorno = false;
        } finally {

            request.disconnect();
        }

        return retorno;
    }

    @Override
    protected void onPostExecute(Boolean result) {

        indicadorDeAtividade.dismiss();

        if (result) {

        } else {
            mostrarErro(0, R.string.erroEnviarCodigo);
        }
    }
}.execute();