The method `plan_hours` does nothing if the `hours` parameter is equal to 0.
This commit adds the 0 case in the logic, and add an optional `resource` in the
parameters.
Example: A workorder works each days between 8am and 4pm. If we call
`plan_hours(0)` at 9pm, we want to get 8am the next day. Currently, it
sends back 9pm.
Before this commit, a hack existed consisting to send a very small value to
`plan_hours`. The holidays and non-working time were taken into account and the
result was rounded to the second. We can now send explicitely 0 and the
non-working hours will be skipped.
William Henrotin
701d0f0e1a