* array.c (rb_ary_dup): make returned array the same class as the original
  array [Bug #7768] [ruby-core:51792]
* test/ruby/test_array.rb (class TestArray): add test

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39004 b2dd03c8-39d4-4d8f-98ff-823fe69b080e