feat(subsonic): return changed field in playlist response

closes #455

server/ctrlsubsonic/handlers_playlist.go

@@ -197,6 +197,7 @@ func playlistRender(c *Controller, params params.Params, playlistID string, play
 		Name:      playlist.Name,
 		Comment:   playlist.Comment,
 		Created:   playlist.UpdatedAt,
+		Changed:   playlist.UpdatedAt,
 		SongCount: len(playlist.Items),
 		Public:    playlist.IsPublic,
 		Owner:     user.Name,

server/ctrlsubsonic/spec/spec.go

@@ -310,6 +310,7 @@ type Playlist struct {
 	Owner     string        `xml:"owner,attr"      json:"owner"`
 	SongCount int           `xml:"songCount,attr"  json:"songCount"`
 	Created   time.Time     `xml:"created,attr"    json:"created"`
+	Changed   time.Time     `xml:"changed,attr"    json:"changed"`
 	Duration  int           `xml:"duration,attr"   json:"duration,omitempty"`
 	Public    bool          `xml:"public,attr"     json:"public,omitempty"`
 	List      []*TrackChild `xml:"entry,omitempty" json:"entry,omitempty"`