allow empty headers on DataFromReader (#2121)

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 = map[string]string{}
+		}
 		r.Headers["Content-Length"] = strconv.FormatInt(r.ContentLength, 10)
 	}
 	r.writeHeaders(w, r.Headers)