encoding.c: encoding check

* encoding.c (enc_check_encoding): use is_data_encoding() to check
  type consistently.
* encoding.c (must_encoding): return rb_encoding* instead of encoding
  index.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41968 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
encoding.c: defer autoload

* encoding.c (enc_inspect): defer loading autoloaded encoding.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41969 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
encoding.c: NO_PRESERVED_ENCODING

* encoding.c (rb_enc_init): no longer needs NO_PRESERVED_ENCODING.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41970 b2dd03c8-39d4-4d8f-98ff-823fe69b080e