Compatibility for ruby 2.0 

fix the error :
`/lib/deep_cloneable.rb:15:in `remove_method': method `dup' not defined in ActiveRecord::Base (NameError)

in the tests.
tests pass on 1.8.7, 1.9.2, 2.0.0 (with rails 2.3.13.rc1)
Change the respond_to? as the rules for respond_to method have changed in ruby 2.0 (http://tenderlovemaking.com/2012/09/07/protected-methods-and-ruby-2-0.html)