To compute start and end date of a working interval this function replaced
hour in datetime object without taking into account the time zone. The start and end date
taking respectively from calendar_working_day.hour_from and calendar_working_day.hour_to are
in the time zone of the user and the datetime object are compared in UTC to avoid schedule's gap.
This is why the start and end must be converted in UTC after being replaced by these hours.
The tz_info must be removed from dates because it's forbidden to compare naive and aware dates.
opw:648349
Regarding _schedule_days, similar thing was done in
saas-6, @ fd5e7f2.
When the argument `days` was negative, the
returned intervals were wrong.
Regarding `get_working_intervals_of_day`,
calling this method without
`default_interval` and calendar
set crashed.
Closes#8208
Christophe Simonis
Goffin Simon
Antonio Espinosa
Xavier Morel
Thibault Delavallée