Handle artist MBIDs as multivalue tags

Instead of joining artist MBIDs with a ';' save them as multiple values
to the tag. (This is how Picard saves that tag.)

Signed-off-by: Frederik “Freso” S. Olesen <freso.dk@gmail.com>

whipper/common/mbngs.py

@@ -143,8 +143,9 @@ class _Credit(list):
                                            i.get('artist').get('name', None)))
 
     def getIds(self):
+        # split()'s the joined string so we get a proper list of MBIDs
         return self.joiner(lambda i: i.get('artist').get('id', None),
-                           joinString=";")
+                           joinString=";").split(';')
 
 
 def _getWorks(recording):