android 微信支付，body为中文字符，签名错误

[code]private String genProductArgs() {
        StringBuffer xml = new StringBuffer();
        try {
            String nonceStr = genNonceStr();
            xml.append("</xml>");
            List<NameValuePair> packageParams = new LinkedList<NameValuePair>();
            packageParams
                    .add(new BasicNameValuePair("appid", Constants.APP_ID));
            packageParams.add(new BasicNameValuePair("body", "测试"));
            packageParams
                    .add(new BasicNameValuePair("mch_id", Constants.MCH_ID));
            packageParams.add(new BasicNameValuePair("nonce_str", nonceStr));
            packageParams.add(new BasicNameValuePair("notify_url",
                    "http://121.40.35.3/test"));
            packageParams.add(new BasicNameValuePair("input_charset", "utf-8"));
            packageParams.add(new BasicNameValuePair("out_trade_no",
                    genOutTradNo()));
            packageParams.add(new BasicNameValuePair("spbill_create_ip",
                    "127.0.0.1"));
            packageParams.add(new BasicNameValuePair("total_fee", "1"));
            packageParams.add(new BasicNameValuePair("trade_type", "APP"));
            String sign = genPackageSign(packageParams);
            packageParams.add(new BasicNameValuePair("sign", sign));
            String xmlstring = toXml(packageParams);
            return xmlstring;
        } catch (Exception e) {
            Log.e(TAG, "genProductArgs fail, ex = " + e.getMessage());
            return null;
        }
    }

[code]String xmlstring = toXml(packageParams);
//改变拼接之后xml字符串格式
return new String(xmlstring.toString().getBytes(), "ISO8859-1");