[FIX] mrp: update production quantity wizard factor

Use the same factor than the one used when
computing the products to consume at confirm.

Before, the quantity of products to consume was wrong
when using a different unit of measure in the BOM
than in the production order.

e.g.
 - BoM defined as:
   - 1 dozen of milk
   - BoM lines: 1 cow
 - Production order:
   - 1 unit of milk
   - BoM: The above one
At confirm, the quantity to consume is 0,083 cow (1 cow / 12, this is correct)
But, when updating the quantity to 2.0,
it updated the quantity of cow to 24.0, instead of 0,16.

opw-669447

addons/mrp/wizard/change_production_qty.py

@@ -69,6 +69,7 @@ def change_prod_qty(self, cr, uid, ids, context=None):
         prod_obj = self.pool.get('mrp.production')
         bom_obj = self.pool.get('mrp.bom')
         move_obj = self.pool.get('stock.move')
+        uom_obj = self.pool.get('product.uom')
         for wiz_qty in self.browse(cr, uid, ids, context=context):
             prod = prod_obj.browse(cr, uid, record_id, context=context)
             prod_obj.write(cr, uid, [prod.id], {'product_qty': wiz_qty.product_qty})
@@ -87,7 +88,7 @@ def change_prod_qty(self, cr, uid, ids, context=None):
                 if not bom_id:
                     raise osv.except_osv(_('Error!'), _("Cannot find bill of material for this product."))
 
-                factor = prod.product_qty * prod.product_uom.factor / bom_point.product_uom.factor
+                factor = uom_obj._compute_qty(cr, uid, prod.product_uom.id, prod.product_qty, bom_point.product_uom.id)
                 product_details, workcenter_details = \
                     bom_obj._bom_explode(cr, uid, bom_point, factor / bom_point.product_qty, [])
                 product_move = dict((mv.product_id.id, mv.id) for mv in prod.picking_id.move_lines)