vm_method.c: clear cache after refine method

* vm_method.c (rb_add_refined_method_entry): clear cache in the
  refined class since refining a method entry is modifying the class.
  [ruby-core:57079] [Bug #8880]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42896 b2dd03c8-39d4-4d8f-98ff-823fe69b080e