title: Odoo 8 @api.onchange function not let update/Created one2many value
body: <p>I am building a module (Odoo 8) , my target is create offers in sale order, this offer can set a fix price for a determinate product or set a gift to zero cost. </p>
  
  <p>I am adding my custom model offer_line, in new tab inside sale order. </p>
  
  <p>Is defined like this:</p>
  
  <pre><code>class OfferSaleOrderLine(models.Model):
  
  _name = 'offer.sale.order.line'
  
  sale_order_ref = fields.Many2one('sale.order',ondelete='set null', string="Sale Order", index=True)
  
  offer_ref = fields.Many2one('offer',ondelete='set null', string="Oferta", index=True)
  
  is_active = fields.Boolean(default=True,string='Activo')
  
  accumulations = fields.Float(digits=(6, 2), help="Acumulaciones")
  
   class SaleOrder(models.Model):
  
  _inherit = 'sale.order'
  
  offers_lines = fields.One2many('offer.sale.order.line','sale_order_ref', string="Lineas de Ofertas")
  </code></pre>
  
  <p>I have a new api onchange method inside sale order:</p>
  
  <pre><code> @api.onchange('offers_lines')
  
  def _onchange_offers_lines(self):
  </code></pre>
  
  <p>I check is offer need apply, and i add to offers_line new lines from this onchange function, like this:</p>
  
  <pre><code>self.offers_lines += self.env['offer.sale.order.line'].new({'is_active': True, 'offer_ref': offer, 'accumulations' : is_offer})
  </code></pre>
  
  <p>This is working perfect, lines is created, added to tab in form and onchange methods is trigger. </p>
  
  <p>But the problem is next, if i try the same with sale order line, no working:</p>
  
  <pre><code>     val = {
  
  'name': gift_line.free_product.name,
  
  'order_id': self.id,
  
  'product_id': gift_line.free_product.id,
  
  'product_uom_qty': gift_line.qty,
  
  'product_uom': self.order_line[0].product_uom.id,
  
  'price_unit': 0.0,
  
  'state': 'draft',
  
  }
  
  self.order_line += self.env['sale.order.line'].new(val)
  </code></pre>
  
  <p>In log, this lines is created, i can see the newid id is created when i foreach self.order_line </p>
  
  <p>****ORDER LINES : ID :  ; Product: product.product(5152,) ; Qty: 6.0 ; Price: 0.0 ;****</p>
  
  <p>but the item is no created in sale order line tab, i dont know why, my custom lines(One2many) is created, but, the sale_order_lines, with same code and one2many field too, is not created. I have the same problem if i try set the price_unit to this sale_order_lines. Log says changes is added, but is not updated in form. In next onchange trigger, the changes is dissapear.</p>
  
  <p>Thanks to all!</p>