Vary headers gets overwritten instead of it being added.

[TerraSkye]

package main

import (
	"fmt"
	"github.com/gin-contrib/gzip"
	"github.com/gin-gonic/gin"
	"log"
	"net/http"
	"time"
)

func main() {

	r := gin.Default()
	r.Use(func(c *gin.Context) {
		c.Writer.Header().Add("Vary", "Origin")
	}, gzip.Gzip(gzip.DefaultCompression))
	r.GET("/ping", func(c *gin.Context) {
		c.String(http.StatusOK, "pong "+fmt.Sprint(time.Now().Unix()))
	})

	// Listen and Server in 0.0.0.0:8080
	if err := r.Run(":9000"); err != nil {
		log.Fatal(err)
	}

}

when running the folowing setup. i expect the Vary headers to contain
Accept-Encoding and Origin

but due to the behavior of the gzip middleware, the Vary header is being overwritten

shouldnt gzip add the the Accept-Encoding instead over overwriting it ?

[runephilosof-karnovgroup]

Also, Vary should also include Accept-Encoding when the request did include Accept-Encoding

runephilosof@fedora:~/code/go-test$ curl -i localhost:9000/ping
HTTP/1.1 200 OK
Content-Type: text/plain; charset=utf-8
Vary: Origin
Date: Wed, 20 Sep 2023 12:09:17 GMT
Content-Length: 15

pong 1695211757runephilosof@fedora:~/code/go-test$ 
runephilosof@fedora:~/code/go-test$ curl -i -H 'Accept-Encoding: gzip' localhost:9000/ping
HTTP/1.1 200 OK
Content-Encoding: gzip
Content-Type: text/plain; charset=utf-8
Vary: Accept-Encoding
Date: Wed, 20 Sep 2023 12:10:09 GMT
Content-Length: 39