- it's its job to handle group-by-resource-by-default behaviour when another group-by is provided
  - move task of updating dictionnary from this method to caller, to make it easier to understand when reading caller


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@16062 20353a03-c40f-0410-a6d1-a30d3c3de9de