AddRequestHeader

添加HTTP请求头的值，它不会覆盖原有值，而是添加。

Example

package main
import (
    "github.com/vicanso/elton"
    "github.com/vicanso/elton/middleware"
)
func main() {
    e := elton.New()
    e.Use(middleware.NewDefaultResponder())
    e.GET("/", func(c *elton.Context) (err error) {
        c.AddRequestHeader("User-Agent", "go-agent")
        c.Body = c.Request.Header
        return
    })
    err := e.ListenAndServe(":3000")
    if err != nil {
        panic(err)
    }
}