Batch update - Can't modify custom fields for a contact with 2 subtypes

When a contact has two subtypes, it is impossible to batch update custom fields via a profile.

Steps to reproduce the bug:

1. Create custom fields (image : batch_update-custom fields.png)
2. Create a profile with this fields + a common fields like Middle Name (image : batch_update-profile.png)
3. Create or use 2 subtypes like Student and Parent. And add this subtype to a contact. In my screen capture, it's Barkley Delana.
4. Modify this contact via the profile. (image : batch_update-before.png)

(This procedure can be tested today on Civicrm demo site with custom fields and profile that i have created : http://dmaster.demo.civicrm.org/civicrm/admin/uf/group?reset=1 )

As you can see, on my screen captures, for the contact with 2 subtypes, custom fields are not updated. (image : batch_update-after.png)