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.