opened
songjiayang
username_0
- go version: go version go1.11.2 darwin/amd64
- gin version:  v1.3.0
- operating system: darwin/amd64

## Description
I use `DataFromReader`  add extraHeaders to response Header,  but the header not format like httpHeader.Set(),  my test code is:

```golang
package main

import (
	"github.com/gin-gonic/gin"
	"strings"
)

func main() {
	r := gin.Default()
	r.GET("/req1", func(c *gin.Context) {
		c.DataFromReader(200, 5,  gin.MIMEMultipartPOSTForm, strings.NewReader("hello"), map[string]string{
			"x-request-id": "id",
		})
	})

	r.GET("/req2", func(c *gin.Context) {
		c.Writer.Header().Set("x-request-id", "id")
		c.DataFromReader(200, 5,  gin.MIMEMultipartPOSTForm, strings.NewReader("hello"), map[string]string{})
	})
	r.Run()
}

```

## Screenshots
<img width="655" alt="screen shot 2018-12-29 at 3 13 10 pm" src="https://user-images.githubusercontent.com/1459834/50534751-561b7100-0b7c-11e9-8e44-a4cbb8d175a8.png">

As your see `req1` response header key is `x-request-id` but  `req2` is `X-Request-Id` and client use `response.Heaer().Get("x-request-id") `  can only get `req2` value. 

The `DataFromReader`'s extraHeaders setting should have the same behavior as `c.Writer.Header().Set`.
gin render.writeHeaders is not MIMEHeader format
issue
closed
thinkerou
username_1
issue