allowance is not being updated when 2**160-1 is approved

[codeislight1]

i was wondering in _transfer whether the bypass of updating the allowed.amount when approved amount is 2**160-1 is intended to save on gas, as i have tried to test in 2 cases, for any value under the max, it did deduct from the allowance accordingly. and also I was looking into the test cases and there is a lack of checking for allowance when permit2.transferFrom takes place.