Don't marshal some fields into .nfo files

There are some fields of the video.Video struct that are for my code
only and should not make it into the actual .nfo file.

video/video.go

@@ -10,22 +10,22 @@ import (
 )
 
 type Video struct {
-	Url          string
 	XMLName      xml.Name `xml:"movie"`
 	Title        string   `xml:"title"`
 	Description  string   `xml:"plot"`
-	DownloadDate string
-	FileName     string
+	url          string
+	downloadDate string
+	fileName     string
 }
 
 func NewVideo(url string) *Video {
 	currentTime := time.Now().Local().Format(time.RFC3339)
 	v := &Video{
-		Url:          url,
+		url:          url,
 		Title:        youtubedl.GetTitle(url),
 		Description:  youtubedl.GetDescription(url),
-		DownloadDate: currentTime,
-		FileName:     fmt.Sprintf("%s %s", currentTime, youtubedl.GetTitleSafe(url)),
+		downloadDate: currentTime,
+		fileName:     fmt.Sprintf("%s %s", currentTime, youtubedl.GetTitleSafe(url)),
 	}
 
 	if v.Title == "" {
@@ -37,16 +37,16 @@ func NewVideo(url string) *Video {
 
 func (v *Video) WriteVideo(directory string) error {
 	return youtubedl.DownloadTo(
-		v.Url,
-		fmt.Sprintf("%s/%s.%s", directory, v.FileName, "%(ext)s"),
+		v.url,
+		fmt.Sprintf("%s/%s.%s", directory, v.fileName, "%(ext)s"),
 	)
 }
 
 // WriteNfo saves a thumbnail file for the video.
 func (v *Video) WriteThumb(directory string) error {
 	return youtubedl.DownloadThumbnailTo(
-		v.Url,
-		fmt.Sprintf("%s/%s-thumb.%s", directory, v.FileName, "%(ext)s"),
+		v.url,
+		fmt.Sprintf("%s/%s-thumb.%s", directory, v.fileName, "%(ext)s"),
 	)
 }
 
@@ -60,10 +60,10 @@ func (v *Video) WriteNfo(directory string) error {
 
 	nfoXml = []byte(fmt.Sprintf("%s\n%s\n%s",
 		`<?xml version="1.0" encoding="UTF-8" standalone="yes"?>`,
-		fmt.Sprintf("<!-- Created on %s by SquirrelBot -->", v.DownloadDate),
+		fmt.Sprintf("<!-- Created on %s by SquirrelBot -->", v.downloadDate),
 		nfoXml,
 	))
-	err = ioutil.WriteFile(fmt.Sprintf("%s/%s.nfo", directory, v.FileName), nfoXml, 0644)
+	err = ioutil.WriteFile(fmt.Sprintf("%s/%s.nfo", directory, v.fileName), nfoXml, 0644)
 	if err != nil {
 		return err
 	}