[FIX] purchase: invoice_open should return action.

This line was wrongly removed by 0fd773a486

Courtesy of @alhashash (http://git.io/vJfRQ)

addons/purchase/purchase.py

@@ -422,6 +422,7 @@ class purchase_order(osv.osv):
         if not inv_ids:
             raise UserError(_('Please create Invoices.'))
         result['domain'] = [('id', 'in', inv_ids)]
+        return result
 
     def view_invoice(self, cr, uid, ids, context=None):
         '''