Add missing write barrier in set_i_initialize_copy

When we copy the table from one set to another we need to run write
barriers.

Author: jhawthorn

set.c

@@ -528,6 +528,7 @@ set_i_initialize_copy(VALUE set, VALUE other)
 
     set_free_embedded(sobj);
     set_copy(&sobj->table, RSET_TABLE(other));
+    rb_gc_writebarrier_remember(set);
 
     return set;
 }

test/ruby/test_set.rb

@@ -129,6 +129,12 @@ def test_replace
     assert_same(set, ret)
     assert_equal(Set['a','b','c'], set)
 
+    set = Set[1,2]
+    ret = set.replace(Set.new('a'..'c'))
+
+    assert_same(set, ret)
+    assert_equal(Set['a','b','c'], set)
+
     set = Set[1,2]
     assert_raise(ArgumentError) {
       set.replace(3)