Add specs for gsub with named captures

Related to #2126
Fix using backreferences in String#gsub

Fixes #2126
Remove dead code in Dir::Glob
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