fix(ext-plugin-post-resp): can't set content-type response header

[soulbird]

Description

Fixes #8388 #8505

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[soulbird]

Awaiting the outcome of the discussion at #8388 (comment)

[juzhiyuan]

LGTM

[cod-soap]

@soulbird
There is still this issue, the latest version of 3.6 uses Golang

还是有这个问题，最新版的3.6，用的golang，

把原有的text/html; charset=utf-8 改为了：text/plain; charset=utf-8
代码中甚至没有做任何的业务逻辑，

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)
}