[FIX] hr_timesheet_sheet - search sheet_id for attendance using date in users timezone. As attendances depends on timezone but timesheet have from and to date instead of datetime we have to change the attendance to the same timezone expected for timesheet. And as a Timesheet belongs to a user, we can expect this timesheet to be in the same timezone as its owner. Maybe a timezone field should be added on timesheet. In anycase it make no sense to have all timesheet forced with start and end date as UTC.
[IMP] hr_timesheet_sheet - remove unused variable
[IMP] hr_timesheet_sheet - simplify and improve readability in search for sheet_ids
[IMP] hr_timesheet_sheet - set default timezone as UTC if employee has none
[FIX] hr_timesheet_sheet - remove useless call to AT TIME ZONE in psql request here it isn't current_date but an utc 'timestamp' date. Need a strong tea even if it isn't utc tea time.
[UPD]
[UPD] resolve conflicts with main branch and fix bug introduced in rev 10015 lp:1313622
[MERGE] [FIX] hr_timesheet_sheet: make it timezone tolerant and avoid errors when comparing datetime and dates, courtesy of Yannick Vaucher (Camptocamp)

The attendance date recieved by the server is in UTC while the user sees it in his timezone. This means that an attendance could be in a timesheet (bounded by dates) for a user but not for the server which would not accept a valid attendance.
The fix will make the check in the user's timezone.

Only the date part of the attendance is kept for comparison as the boundaries are dates objects.