Fix segmentation fault in Ruby 2.0.0-p353.

In Ruby 2.0.0-p353 there was a
[commit](https://github.com/ruby/ruby/commit/66915c507777c5e3a978fa73de25db763efd9206)
that switched case matching from actual sending `===` method to magic lookup,
that does not see it in `method_missing`. It's hard to predict how exactly and
when exactly this bug will be solved so here I suggest a solution of defining
it in Duration directly.

In Ruby 2.0.0-p353 without the added fix added test crashes to segmentation
fault.