为什么杜松子酒（golang）使我的自定义标头变成小写？

I'm stuck on this problem while learning go/gin. When I use custom header gin makes it lowercase, here is code example:

package main

import "github.com/gin-gonic/gin"

func main() {
    r := gin.New()
    r.GET("/ping", func(c *gin.Context) {
        c.Header("X-Next", "abc")
        c.String(200, "ok")
    })
    r.Run(":9000")
}

Header becomes x-next, can't figure it out why it is happening. Similar example with standard http module works fine.

Looks like its browser changes case of header, and it is not an issue with gin. Thanks @elithrar for hint.