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