Add and adjust specs on Kernel#initialize_clone for 2.0
Fix a spec on Kernel#initialize_dup and added specs for 2.0
Add specs for Kernel#respond_to_missing?
Fix builtin {initialize_{dup,clone},respond_to_missing?} for 2.0
Merge pull request #2483 from kachick/2.0-builtin_privatized_method

Builtin initialize_{dup,clone} and respond_to_missing? should be private methods as user defined from 2.0