Fix a bug in rb_include_module that stops nested inclusion into module subclasses
This bug was present since the code was originally added by me
in 3556a834a2847e52162d1d3302d4c64390df1694.
Fixes [Bug #20871]
Related issues
Bug #20871: Including methods in Enumerable doesn't make them available in Array
merge revision(s) 3b7892b6e4d1a1a5d6019987f9b46ed443dd104f: [Backport #20871]