bug: POST request with body contain chinese characters exist encoding problem

### Issue description

### Environment

apisix-java-plugin-runner version：0.2.0
apsix version：2.13.1

### Minimal test code / Steps to reproduce the issue

1. Test post request contain chinese characters in OKHttp：
`{"source":"高德地图","content":"https://surl.amap.com/1PoSAsa1l4Ud","deviceId":"123456"}`
2. inner OKHttp，body will be translated to utf-8 format bytes:
`Charset charset = Util.UTF_8;
byte[] bytes = content.getBytes(charset);
`
4. Custom filter in apisix-java-plugin-runner, get request body:
`{"source":"高德地图","content":"https://surl.amap.com/1PoSAsa1l4Ud","deviceId":"123456"}`