Set Timeouts should update existing timeouts, not overwrite them all

[shs96c]

It is unclear when reading the spec if Set Timeouts should reset all timeouts to their default values and only update a single value if called with only one value (eg. script), or whether it should only alter the given value.

All existing implementations choose the latter implementation, which is also the one that intuitively feels correct. I suggest we amend the spec to make this clear.