make musicFolderIds ints (#169)

* make `musicFolderId`s ints

https://github.com/sentriz/gonic/issues/50#issuecomment-963257103
006c1dafee/Classes/Models/API%20Models/MediaFolder.swift (L16)

related #50

* add a test for multi folders and reused artists

server/ctrlsubsonic/ctrl.go

@@ -30,6 +30,7 @@ type Controller struct {
 	CachePath      string
 	CoverCachePath string
 	PodcastsPath   string
+	MusicPaths     []string
 	Jukebox        *jukebox.Jukebox
 	Scrobblers     []scrobble.Scrobbler
 	Podcasts       *podcasts.Podcasts
@@ -116,3 +117,14 @@ func (c *Controller) HR(h handlerSubsonicRaw) http.Handler {
 		}
 	})
 }
+
+func (c *Controller) getMusicFolder(p params.Params) string {
+	idx, err := p.GetInt("musicFolderId")
+	if err != nil {
+		return ""
+	}
+	if idx < 0 || idx > len(c.MusicPaths) {
+		return ""
+	}
+	return c.MusicPaths[idx]
+}