Originally, time object is decorated after `changes_applied` by
`forgetting_assignment` (`value_for_database`).

https://github.com/rails/rails/blob/d2cdf0be675b44771f950697fc0b19ef0ea453f9/activemodel/lib/active_model/attribute.rb#L67-L69
https://github.com/rails/rails/blob/d2cdf0be675b44771f950697fc0b19ef0ea453f9/activemodel/lib/active_model/attribute.rb#L55-L57

Before #36352, fortunately `apply_seconds_precision` will return
undecorated new time object by everytime `value.change` is called.

Now, we need to explicitly check a time value is decorated or not in
`cast_value`.

Fixes #36910.