[FIX] web: read_progress_bar with complex group_by

The naive implementation of read_progress_bar() does not deal properly
with groupings like `date:week`.  The values in the groups are returned
under the wrong key ('date' instead of 'date:week').

closes odoo/odoo#73477

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>