array.c: set class

* array.c (rb_ary_uniq): set class of the return value to the receiver
  class.  fix failure in TestArray#test_array_subclass.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43196 b2dd03c8-39d4-4d8f-98ff-823fe69b080e