Fix `ActiveSupport::Duration#instance_of?` to behave like `#is_a?`

Hello,

This pull request is a follow-up to #16560. The latter introduced a regression as `ActiveSupport::Duration#instance_of?` should return true with `Fixnum` to make this layer "transparent" as people most likely consider `1.day` as a fixnum. Thus this pull request makes `#instance_of?` behave like `#is_a?` as this was previously returning false for the class itself since it was delegated to the `value` attribute.

A second commit has also been attached to skip the `#eql?` tests on Rubinius since the first purpose of #16560 was to make those tests to pass.

Have a nice day.
@robin850 should this be reported as a bug to rubinius?