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.

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;
 }