[ruby/set] Fix ^ to respect subclasses

https://github.com/ruby/set/commit/f88ecdef6b

lib/set.rb

@@ -662,7 +662,7 @@ class Set
   #     Set[1, 2] ^ Set[2, 3]                   #=> #<Set: {3, 1}>
   #     Set[1, 'b', 'c'] ^ ['b', 'd']           #=> #<Set: {"d", 1, "c"}>
   def ^(enum)
-    n = Set.new(enum)
+    n = self.class.new(enum)
     each { |o| n.add(o) unless n.delete?(o) }
     n
   end

test/set/test_set.rb

@@ -643,6 +643,11 @@ class TC_Set < Test::Unit::TestCase
     ret = set ^ [2,4,5,5]
     assert_not_same(set, ret)
     assert_equal(Set[1,3,5], ret)
+
+    set2 = Set2[1,2,3,4]
+    ret2 = set2 ^ [2,4,5,5]
+    assert_instance_of(Set2, ret2)
+    assert_equal(Set2[1,3,5], ret2)
   end
 
   def test_eq