7.0 bug compute qty same uom

Case:

* Create a new unit 6kg based on kg product uom category with factor 6 and rounding 1.0;
* Call the function _compute_qty of product_uom Model with the folllowing parameters:
 * from_unit = to_unit = 6kg
 * qty = 1

* Expected value: 1 (the input value, because there is no conversion to do).
* Return value: 0. (depending of rounding setting).

The case is written in a TransactionCase in test_uom.py

**Classical User Case**:
when an Delivery order is validated, the function _compute_qty is called, even if the units are identical.