Add a spec for defining `initialize_dup` from 2.0
Add a spec for defining `initialize_clone` from 2.0
Add a spec for defining `respond_to_missing?` from 2.0
Fix to define initialie_{dup,clone} and respond_to_missing? for 2.0
Merge pull request #2479 from kachick/2.0-initialzie_dup,clone

Initialize_{dup,clone} and respond_to_missing? should automatically become a private method from 2.0