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.
The test has been moved under `test_is_a`. Sorry actually I mess between `value.is_a?(klass)` and `value.instance_of?(klass)`. ActiveSupport::Duration's `#instance_of?` should not behave like `#is_a?` ; it should just return true for itself and Fixnum.

Thanks for your review ; good call ! :-)