Forum rates (final aggregate grade) aren't moved to gradebook

I was testing MDL-17364, when I've notice that forum grades (aggregation of rates) aren't being moved properly to the gradebook.

I've done this:

1) Create course.
2) Create general forum
3) Override mod:forum/rate to students
4) Student A creates a post.
5) Various students rate that post

Expected behaviour:

a) Grade of forum (aggregation of rates) is properly calculated and displayed in the forum.
b) That grade is copied to the gradebook and available there.

Current behaviour:

a) Works ok
b) Grade isn't copied to the gradebook.

Note that same steps are working ok in MOODLE_19_STABLE. I only have been able to trace the problem down to the forum_update_grades() that seems to be noticeable different in 19_STABLE and HEAD.

Ciao