[FIX] Qty rounding when UOS are same

There is no need to perform operations (from line 185) that can lead to rounding issues if UOM conversion *From* and *To* are same.

## Scenario that leads to issue
### Reference unit: 
name: Unit
factor: 1
rounding: 0,01

### Smaller then reference unit: 
name: 1/8 Unit
factor: 8
rounding: 0,01

### Test Case

If you pass same UOM, e.g **1/8 Unit** to from_unit and to_unit (stock_partial_picking.py:193) and qty will be 0.01 _compute_qty_obj will return 0.008 what is wrong.

There can be and problem with ceiling() too, but I didn't invest time to check it.