[ruby/forwardable] Fix NoMethodError on ruby 2.4 or earlier
https://github.com/ruby/forwardable/runs/242918994#step:5:12
Error: test_obj_single_delegators_send_id(TestForwardable): NoMethodError: private method `attr_reader' called for #<Class:#<Object:0x00005605af501f58>>
https://github.com/ruby/forwardable/commit/711bbb2466
Added by znz (Kazuhiro NISHIYAMA) about 6 years ago
[ruby/forwardable] Fix NoMethodError on ruby 2.4 or earlier
https://github.com/ruby/forwardable/runs/242918994#step:5:12
https://github.com/ruby/forwardable/commit/711bbb2466