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.
I'm not sure that they can do something on their end but I may be wrong. `Numeric#eql?` will call [`#instance_of?`](https://github.com/rubinius/rubinius/blob/90a40b05e16bfe2c5d2d1f875c7009bb49e9e348/kernel/common/numeric.rb#L22) on the given object. The problem is that since Rubinius is written in Ruby, when we are dealing with core objects, we should try to follow as much as possible the vanilla behavior ; this  is certainly another case of [Rubinius wants to help you make Ruby better](https://blog.engineyard.com/2010/rubinius-wants-to-help-you-make-ruby-better).

In my opinion, this is something to fix on our end for 5.0. At the moment, `#instance_of?` doesn't behave in an idiomatic way ; we may have to completely refactor this layer (unless we simply don't want to be BC with this method in 5.0).