* class.c (rb_prepend_module): make T_ICLASS object shady because
  this T_ICLASS object seems to share method table with other class
  objects. It was causes WB miss.
  TODO: need to know the data structure.
* test/ruby/test_module.rb: add a test for WB miss.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42534 b2dd03c8-39d4-4d8f-98ff-823fe69b080e