negative remaining_hours in project_task

As soon as the effectivly done work on a task in hours exceeds the planned hours I'd expect the delay_hours to reflect that situation. However instead the remaining_hours will further decreased turning into a negative number.

steps to reproduce:
1. create a task with 0-X planned hours (0 to see the effect straight away)
2. do some work on the task (less then X)
3. see remaining hours decrease
4. Up to now: everything fine
5. do some work so the effective hours exceed the plannend hours(X)
6. Remaining hours turn negative

how to fix:
* IMO reamining_hours must never be less than 0
* when saving the work and recalculation is done remaining hours should only be lowered to minimum 0

project/project.py
class project_work(osv.osv)
def write
 cr.execute('update project_task set remaining_hours=remaining_hours - %s + (%s) where id=%s', (vals.get('hours',0.0), work.hours, work.task_id.id))