array.c: return first unique element in Array#uniq

* array.c (ary_add_hash): Fix consistency issue between Array#uniq and
  Array#uniq! [Bug #9340] [ruby-core:59457]
* test/ruby/test_array.rb (class TestArray): regression test for above.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44512 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-01-07

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44513 b2dd03c8-39d4-4d8f-98ff-823fe69b080e