Make compatible with Ruby 2.4

rm155 · hsbt · commit 0a77bb492d16 · 2024-10-01T17:26:35.000+09:00
diff --git a/lib/singleton.rb b/lib/singleton.rb
@@ -156,7 +156,7 @@ def self.included(c)
 
     def self.extended(c)
       # extending an object with Singleton is a bad idea
-      c.singleton_class.undef_method :extend_object
+      c.singleton_class.send(:undef_method, :extend_object)
     end
 
     def __init__(klass) # :nodoc:
