Fix inconsistent typing of duration properties

[MaxAake]

When creating a Duration the seconds and nanoseconds will be converted to Integers, no matter if provided with Integers, Numbers, or BigInts. This is awkward and requires special handling by users

[robsdedude]

Optional nit: the fact that your last commits changed behavior while all tests stayed the same (and passed before and still do) suggests that test coverage could be improved :)