Unable to change TVA percentage

OpenERP Version: 7.0-20130520-231036

To reproduce this bug, you do the following:

1. create a quotation for 1 quotation line where you do not refer to any product but only give an description of your sale, a fixed amount and a tax percentage (TVA) of 0%.
2. confirm the sale and create a sales order without modifications, the tax percentage and other values remain
3. convert the sales order to an invoice and in the concept invoice, you try to modify the tax percentage to 21
4. after saving this modification, you will see the tax is not recalculated and remains zero (0) as it was before

Furthermore the OpenERP version works fine also creating new invoices, sales orders with a reference to a product works as expected.