object.c: error message encoding

* object.c (convert_type, rb_convert_type, rb_check_convert_type),
  (rb_to_integer): preserve class name encoding error messages.

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44758 b2dd03c8-39d4-4d8f-98ff-823fe69b080e