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/spec/spec.go

@@ -179,7 +179,7 @@ type MusicFolders struct {
 }
 
 type MusicFolder struct {
-	ID   string `xml:"id,attr,omitempty"   json:"id,omitempty"`
+	ID   int    `xml:"id,attr"             json:"id"`
 	Name string `xml:"name,attr,omitempty" json:"name,omitempty"`
 }