DataFromReader with nil extraHeaders crashes

[nikandfor]

Description

gin.Context.DataFromReader crashes if I don't set extra headers.

I think this little fix would be the best solution to keep API and not to compel me to write map[string]string{} each time.

diff --git a/render/reader.go b/render/reader.go
index 502d939..fd308a7 100644
--- a/render/reader.go
+++ b/render/reader.go
@@ -22,6 +22,9 @@ type Reader struct {
 func (r Reader) Render(w http.ResponseWriter) (err error) {
        r.WriteContentType(w)
        if r.ContentLength >= 0 {
+               if r.Headers == nil {
+                       r.Headers = make(map[string]string)
+               }
                r.Headers["Content-Length"] = strconv.FormatInt(r.ContentLength, 10)
        }
        r.writeHeaders(w, r.Headers)

I also have question why extra headers does not replace global headers here?
https://github.com/gin-gonic/gin/blob/master/render/reader.go#L41

// render/reader.go

// writeHeaders writes custom Header.
func (r Reader) writeHeaders(w http.ResponseWriter, headers map[string]string) {
    header := w.Header()
    for k, v := range headers {
        if header.Get(k) == "" {
            header.Set(k, v)
        }
    }
}

How to reproduce

package main

import (
    "net/http"
    "strings"

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

func main() {
    r := gin.Default()

    r.GET("/", func(c *gin.Context) {
        // Why not set extra headers as usual (as c.JSON or c.Data)?
        c.Writer.Header().Set("X-Extra-Header", "extra value")

        c.DataFromReader(http.StatusOK, 5, gin.MIMEHTML, strings.NewReader("1234\n"), nil) // If I don't want to set extra headers it crashes
    })

    r.Run(":5000")
}

Expectations

Successful response

$ curl -v http://localhost:5000
# ...
< HTTP/1.1 200 OK
# ...

Actual result

recovered panic and 500 HTTP response

$ curl -v http://localhost:5000
# ...
< HTTP/1.1 500 Internal Server Error
# ...

Environment

• go version: go1.13 linux/amd64
• gin version (or commit ref): v1.4.0 and master
• operating system: ubuntu