bug: pkgHTTP.Request RespHeader() Add override previous value

### Issue description
```
w http.ResponseWriter, r pkgHTTP.Request
r.RespHeader().Add("Set-Cookie","a=a; Expires=Wed, 21 Oct 2023 07:28:00 GMT; Secure; HttpOnly")
r.RespHeader().Add("Set-Cookie","b=b; Expires=Wed, 21 Oct 2023 07:28:00 GMT; Secure; HttpOnly")
```

Cookie `a` is missing in the response header

### Environment

* APISIX Go Plugin Runner's version: dee7fa0167af0ed8cdcd87d4321a43b194f2a7ed
* APISIX version: 2.13.0
* Go version: 1.18
* OS (cmd: `uname -a`): centos

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

1. Start APISIX on docker
2. Set Plugin filter codes
```
Filter(conf interface{}, w http.ResponseWriter, r pkgHTTP.Request)
{
r.RespHeader().Add("Set-Cookie","a=a; Expires=Wed, 21 Oct 2023 07:28:00 GMT; Secure; HttpOnly")
r.RespHeader().Add("Set-Cookie","b=b; Expires=Wed, 21 Oct 2023 07:28:00 GMT; Secure; HttpOnly")
return
}

```
4. Cookie `a` is missing in the response header

### What's the actual result? (including assertion message & call stack if applicable)

Cookie `b` can be found, Cookie `a` is missing

### What's the expected result?
Both cookie `a` and `b` could be found