marshal.c: no duplicated encoding

* marshal.c (w_object): do not dump encoding which is dumped with
  marshal_dump data.  [ruby-core:54334] [Bug #8276]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40366 b2dd03c8-39d4-4d8f-98ff-823fe69b080e