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)



import time
from datetime import datetime, timedelta
from dateutil import relativedelta
import babel
from odoo import api, fields, models, tools, _
from odoo.exceptions import UserError, ValidationError
from odoo.tools.safe_eval import safe_eval
from odoo.addons import decimal_precision as dp
class HrPayrollStructure(models.Model):
    """
    Salary structure used to defined
    - Basic
    - Allowances
    - Deductions
    """
    _name = 'hr.payroll.structure'
    _description = 'Salary Structure'
    @api.model
    def _get_parent(self):