hash.c: reject should return a plain hash

* hash.c (rb_hash_reject): return a plain hash, without copying
  the class, default value, instance variables, and taintedness.
  they had been copied just by accident.
  [ruby-core:59045] [Bug #9223]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44137 b2dd03c8-39d4-4d8f-98ff-823fe69b080e