request help: Request header modification sometimes does not take effect in ext-plugin-pre-req

### Issue description
The client uses `Content-Type:application/x-www-form-urlencoded ` to access the HTTP API. After decrypting the body to json, I reset the request header by `r.Header().Set("Content-Type", "application/json;charset=UTF-8")`, but found that it sometimes does not work.

I have attempted to request 100 times, but there are 9 cases where it does not take effect

the code of plugin:
```go
r.Header().Set("Content-Type", "application/json;charset=UTF-8")
r.SetBody(origBodyByte)

// pluginLogger is a wrapper for log that can output traceId
pluginLogger.Infof("request get Content-Type: %s", r.Header().Get("Content-Type"))
```
the log of the plugin:

```
2024/01/09 20:15:00 [warn] 302#302: *913 [lua] init.lua:961: 2024-01-09T20:15:00.941+0800 INFO pluginlogger/logger.go:34
[00-3334f5bcb64c9c9cdf0da1997a9a81b6-5ff503ca821603ae-01] request get Content-Type: application/json;charset=UTF-8
```
the log of http api:

```
2024-01-09 20:15:00.946 [3334f5bcb64c9c9cdf0da1997a9a81b6] [ http-nio-5001-exec-7:17758181 ] - [ DEBUG ]- [ org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor ] Read "application/x-www-form-urlencoded;charset=UTF-8" to ["**********"]

2024-01-09 20:15:00.946 [3334f5bcb64c9c9cdf0da1997a9a81b6] [ http-nio-5001-exec-7:17758181 ] - [ DEBUG ]- [ com.xxxx.controller.TestController ] Content-Type:application/x-www-form-urlencoded
```

Is there anyone who can help explain the reason?

### Environment

* APISIX Go Plugin Runner's version:
github.com/apache/apisix-go-plugin-runner v0.5.1-0.20231128010119-a265bcd63ca1
* APISIX version: 3.6.0
* Go version: 1.19
* OS (cmd: uname -a): debian:bullseye-slim

Start with the shown Go plugin code and the APISIX Go Plugin Runner v0.5.1 environment, then reproduce the request 100 times while comparing the plugin and HTTP API logs. Investigate why the logged Content-Type differs from the value received by the API, and document a reproducible cause or confirmed fix.

Written by the indexing model from the issue text.