title: AttributeError: 'bool' object has no attribute 'strftime'
body: <p>I am inheriting 'account.partner.ledger' module. When we select the customer we will be able to print the report of the customer's ledger. In the partner ledger menu I want to make 'include Initial Balances' checkbox checked by default if the filter is by date/period.I tried to override the method by my custom module but I am unable to solve the error which I am getting.</p>
  
  <p>Code,</p>
  
  <pre><code>@api.multi
  def onchange_filter(self,filter='filter_no', fiscalyear_id=False):
      res = super(account_partner_ledger, self).onchange_filter(filter=filter, fiscalyear_id=fiscalyear_id)
      if filter in ['filter_no', 'unreconciled']:
          if filter == 'unreconciled':
              res['value'].update({'fiscalyear_id': False})
          res['value'].update({'initial_balance': False, 'period_from': False, 'period_to': False, 'date_from': False ,'date_to': False})
  
      if filter in ['filter_date','filter_period']:
          res['value'].update({'initial_balance': True, 'period_from': True, 'period_to': True, 'date_from': True ,'date_to': True})
      return res
  </code></pre>
  
  <p>Error,</p>
  
  <pre><code>Traceback (most recent call last):
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\http.py", line 544, in _handle_exception
      return super(JsonRequest, self)._handle_exception(exception)
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\http.py", line 581, in dispatch
      result = self._call_function(**self.params)
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\http.py", line 317, in _call_function
      return checked_call(self.db, *args, **kwargs)
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\service\model.py", line 118, in wrapper
      return f(dbname, *args, **kwargs)
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\http.py", line 314, in checked_call
      return self.endpoint(*a, **kw)
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\http.py", line 810, in __call__
      return self.method(*args, **kw)
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\http.py", line 410, in response_wrap
      response = f(*args, **kw)
    File "C:\Users\zendynamix\odooGit\odoo8\addons\web\controllers\main.py", line 944, in call_kw
      return self._call_kw(model, method, args, kwargs)
    File "C:\Users\zendynamix\odooGit\odoo8\addons\web\controllers\main.py", line 936, in _call_kw
      return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs)
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\api.py", line 268, in wrapper
      return old_api(self, *args, **kwargs)
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\api.py", line 399, in old_api
      result = method(recs, *args, **kwargs)
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\models.py", line 5985, in onchange
      record._onchange_eval(name, field_onchange[name], result)
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\models.py", line 5883, in _onchange_eval
      self.update(self._convert_to_cache(method_res['value'], validate=False))
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\models.py", line 5391, in _convert_to_cache
      for name, value in values.iteritems()
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\models.py", line 5392, in &lt;dictcomp&gt;
      if name in fields
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\fields.py", line 1250, in convert_to_cache
      return self.to_string(value)
    File "C:\Users\zendynamix\odooGit\odoo8\openerp\fields.py", line 1240, in to_string
      return value.strftime(DATE_FORMAT) if value else False
  AttributeError: 'bool' object has no attribute 'strftime'
  </code></pre>