Backport r44370 to revert Hash#reject behavior
In [Bug #9223], naruse-san rejected incompatible change due to HashWithIndifferentAccess.
r44358 had changed constant name, but there was one
ifdef condition still use old constant name.
r44370 fixes it to correct one, but it's not backported to 2.1.1.
So HashWithIndifferentAccess in Ruby 2.1.1 is broken because Ruby 2.1.1's Hash#reject doesn't copy extra states.