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)



return self.write({'state': 'draft'})
    @api.multi
    def close_payslip_run(self):
        return self.write({'state': 'close'})
class HrPayslip(models.Model):
    _name = 'hr.payslip'
    _description = 'Pay Slip'
    struct_id = fields.Many2one('hr.payroll.structure', string='Structure',
        readonly=True, states={'draft': [('readonly', False)]},
        help='Defines the rules that have to be applied to this payslip, accordingly '
             'to the contract chosen. If you let empty the field contract, this field isn\'t '
             'mandatory anymore and thus the rules applied will be all the rules set on the '
             'structure of all contracts of the employee valid for the chosen period')
    name = fields.Char(string='Payslip Name', readonly=True,
        states={'draft': [('readonly', False)]})