Adding method `#eql?` to `ActiveSupport::Duration`.

Currently, the following returns `false`, contrary to expectation:

    1.minute.eql?(1.minute)

Adding method `#eql?` will make this behave like expected. Method `#eql?` is
just a bit stricter than `#==`, as it checks whether the argument is also a
duration. Their parts may be different though.

    1.minute.eql?(60.seconds)  # => true
    1.minute.eql?(60)          # => false