encoding.c: is_obj_encoding

* encoding.c (is_obj_encoding): new macro to check if obj is an
  Encoding.  obj can be any type while is_data_encoding expects T_DATA
  only.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42166 b2dd03c8-39d4-4d8f-98ff-823fe69b080e