Add spec for remove_method against cloned singleton.
Add spec for remove_method against cloned singleton.

See jruby/jruby#4228

sgarciac/spec,kachick/rubyspec,kachick/rubyspec,sgarciac/spec,ruby/spec,ruby/rubyspec,nobu/rubyspec,ruby/spec,ruby/rubyspec,nobu/rubyspec,sgarciac/spec,ruby/spec,eregon/rubyspec,eregon/rubyspec,kachick/rubyspec,eregon/rubyspec,nobu/rubyspec