Case merge doesn't copy custom fields over

I can't confirm yet for 4.3 and the demo seems to have a permission problem for CiviCase, but I looked at the code and it seems the same as for 4.2, which is where I'm seeing this is happening.

The code in CRM_Case_BAO_Case::mergeCases should either call CRM_Case_Page_Ajax::_convertToCaseActivity (which is what is used by file on case, copy to case, etc), or because the logic around merging is maybe a little different, it should at least call CRM_Activity_BAO_Activity::copyExtendedActivityData to copy custom fields and attachments.