hash.c: copied identhash

* hash.c (rb_hash_initialize_copy): copy st_table type even if empty.
  [ruby-core:56256] [Bug #8703]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42227 b2dd03c8-39d4-4d8f-98ff-823fe69b080e