Impacted versions: Odoo 10

Steps to reproduce: 
1. Create a contract for the Employee and select the Working Schedule as Day shift.

2. Go to Payroll -> Employee Payslips -> Create. Select Employee and the Contract, Period must be automatically selected. In Worked days, the WORK100 shows no of days and hours by calculating the period and Working schedule.

3. Now try selecting the previous month(1st day to last day). And the worked Days will miscalculate the no of days and hours.(Check Screenshots).

Current behavior: Miscalculation in Worked Days( No of days and Hours) if the period is change e.g. Select a last month

Expected behavior: No of days and hours must be calculated properly after changing the period.

Video/Screenshot link (optional):
![payroll_period](https://cloud.githubusercontent.com/assets/14011478/22732830/a449a954-ee15-11e6-8909-9d714d306bdb.png)
![payroll_hours](https://cloud.githubusercontent.com/assets/14011478/22732819/9e7c34e2-ee15-11e6-9924-a8dc42ce9a5a.png)



locale = self.env.context.get('lang', 'en_US')
        self.name = _('Salary Slip of %s for %s') % (employee.name, tools.ustr(babel.dates.format_date(date=ttyme, format='MMMM-y', locale=locale)))
        self.company_id = employee.company_id
        if not self.env.context.get('contract') or not self.contract_id:
            contract_ids = self.get_contract(employee, date_from, date_to)
            if not contract_ids:
                return
            self.contract_id = self.env['hr.contract'].browse(contract_ids[0])
        if not self.contract_id.struct_id:
            return
        self.struct_id = self.contract_id.struct_id
        
        worked_days_line_ids = self.get_worked_day_lines(contract_ids, date_from, date_to)
        worked_days_lines = self.worked_days_line_ids.browse([])
        for r in worked_days_line_ids:
            worked_days_lines += worked_days_lines.new(r)
        self.worked_days_line_ids = worked_days_lines
        input_line_ids = self.get_inputs(contract_ids, date_from, date_to)
        input_lines = self.input_line_ids.browse([])
        for r in input_line_ids:
            input_lines += input_lines.new(r)
        self.input_line_ids = input_lines
        return
    @api.onchange('contract_id')
    def onchange_contract(self):
        if not self.contract_id:
            self.struct_id = False
        self.with_context(contract=True).onchange_employee()
        return
class HrPayslipWorkedDays(models.Model):