[FIX]point of sale: passing analytic account

In the method action_invoice, the call to the method "_prepare_analytic_account" in dict "inv_line"
had no effect because the value of account_analytic_id was being cleared by the product_id_change statement.
So in the end the invoice line was created without any account attached
Now it checks for an existing value, and if it is not already filled in,
the value from  _prepare_analytic_account is taken.

addons/point_of_sale/point_of_sale.py

@@ -990,14 +990,17 @@ def action_invoice(self, cr, uid, ids, context=None):
                     'invoice_id': inv_id,
                     'product_id': line.product_id.id,
                     'quantity': line.qty,
-                    'account_analytic_id': self._prepare_analytic_account(cr, uid, line, context=context),
                 }
                 inv_name = product_obj.name_get(cr, uid, [line.product_id.id], context=context)[0][1]
                 inv_line.update(inv_line_ref.product_id_change(cr, uid, [],
                                                                line.product_id.id,
                                                                line.product_id.uom_id.id,
                                                                line.qty, partner_id = order.partner_id.id,
                                                                fposition_id=order.partner_id.property_account_position.id)['value'])
+                if not inv_line.get('account_analytic_id', False):
+                    inv_line['account_analytic_id'] = \
+                        self._prepare_analytic_account(cr, uid, line,
+                                                       context=context)
                 inv_line['price_unit'] = line.price_unit
                 inv_line['discount'] = line.discount
                 inv_line['name'] = inv_name