vm_method.c: quote method name

* vm_method.c (set_method_visibility): quote unprintable method name.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40922 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
no clearing method cache

* variable.c (set_const_visibility): return without clearing method
  cache if no arguments.
* vm_method.c (set_method_visibility): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40923 b2dd03c8-39d4-4d8f-98ff-823fe69b080e