Follow-up to #16560

For the sake of backward-compatibility, we need to make #instance_of?
return true for Fixnum. On the other hand, the method should still
give true for ActiveSupport::Duration itself which was not the case
before.
Skip #eql? tests on Rubinius for AS::Duration

Since Rubinius is relying on #instance_of? for its definition of #eql?
(http://git.io/MtmbbA) but ActiveSupport::Duration should behave like
is_a? it returns true with `Fixnum`.

Thus, for the moment, the last assertion is failing so we have to skip
this test.
Merge pull request #16662 from robin850/duration-instance-of

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