fix id3v23 tag writing

Fix due to Chris Cogburn on the mailing list.

beets/mediafile.py

@@ -942,7 +942,9 @@ class MediaFile(object):
                 # In case this is an MP3 object, not an ID3 object.
                 id3 = id3.tags
             id3.update_to_v23()
-        self.mgfile.save()
+            self.mgfile.save(v2_version=3)
+        else:
+            self.mgfile.save()
 
     def delete(self):
         """Remove the current metadata tag from the file.

docs/changelog.rst

@@ -37,6 +37,8 @@ Little fixes:
   reliably (i.e., fewer "no tempo found" messages). Thanks to Peter Schnebel.
 * :doc:`/plugins/convert`: Fix an "Item has no library" error when using the
   ``auto`` config option.
+* Fix an issue that caused the :ref:`id3v23` option to work only occasionally.
+
 
 New stuff: