ext-plugin-post-resp processing content-type have problem

[cod-soap]

Issue description

ext-plugin-post-resp processing content-type have problem

I have seen fixed the issue: apache/apisix#8588
But still have problem

Environment

• APISIX Go Plugin Runner's version: 0.5.0
• APISIX version: 3.6.0
• Go version: 1.19.0
• OS (cmd: uname -a): Linux asec 5.15.0-78-generic 85-Ubuntu SMP Fri Jul 7 15:25:09 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux
  （running in docker）

Minimal test code / Steps to reproduce the issue

1.plugin run in debug mode

2.Changed the original text/html; charset=utf-8 to: text/plain; charset=utf-8
The code doesn't even do any business logic：

type FileRetentionConfig struct {
	Enable bool `json:"enable"`
}

type FileRetentionPlugin struct {
}

func (f FileRetentionPlugin) Name() string {
	return "file_retention_plugin"
}

func (f FileRetentionPlugin) ParseConf(in []byte) (conf interface{}, err error) {
	config := FileRetentionConfig{}
	err = jsoniter.Unmarshal(in, &config)
	return config, err
}

func (f FileRetentionPlugin) RequestFilter(conf interface{}, w http.ResponseWriter, r pkgHTTP.Request) {
	// do nothing
}

func (f FileRetentionPlugin) ResponseFilter(conf interface{}, w pkgHTTP.Response) {
	contentType := w.Header().Get("content-type")
	w.Header().Set("content-type", contentType)
	log.Infof("contentType:%v", contentType)
}

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

hanged the original text/html; charset=utf-8 to: text/plain; charset=utf-8

What's the expected result?

do not change content-type or change to the value specified in the code

[scoolor]

lost content-type?