* hash.c (rb_hash_flatten): fix behavior of flatten(0).
  [ruby-dev:47988] [Bug #9533]

* test/ruby/test_array.rb: test for above. patch is from
  Takeshi Sasaki.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45059 b2dd03c8-39d4-4d8f-98ff-823fe69b080e