Make Object#singleton_methods work correctly for singleton classes of objects

Fixes [Bug #10901]

class.c

@@ -1447,6 +1447,9 @@ rb_obj_singleton_methods(int argc, const VALUE *argv, VALUE obj)
     int recur = TRUE;
 
     if (rb_check_arity(argc, 0, 1)) recur = RTEST(argv[0]);
+    if (RB_TYPE_P(obj, T_CLASS) && FL_TEST(obj, FL_SINGLETON)) {
+        rb_singleton_class(obj);
+    }
     klass = CLASS_OF(obj);
     origin = RCLASS_ORIGIN(klass);
     me_arg.list = st_init_numtable();

test/ruby/test_object.rb

@@ -865,6 +865,29 @@ class TestObject < Test::Unit::TestCase
     assert_match(/@\u{3046}=6\b/, x.inspect)
   end
 
+  def test_singleton_methods
+    assert_equal([], Object.new.singleton_methods)
+    assert_equal([], Object.new.singleton_methods(false))
+    c = Class.new
+    def c.foo; end
+    assert_equal([:foo], c.singleton_methods - [:yaml_tag])
+    assert_equal([:foo], c.singleton_methods(false))
+    assert_equal([], c.singleton_class.singleton_methods(false))
+    c.singleton_class.singleton_class
+    assert_equal([], c.singleton_class.singleton_methods(false))
+
+    o = c.new.singleton_class
+    assert_equal([:foo], o.singleton_methods - [:yaml_tag])
+    assert_equal([], o.singleton_methods(false))
+    o.singleton_class
+    assert_equal([:foo], o.singleton_methods - [:yaml_tag])
+    assert_equal([], o.singleton_methods(false))
+
+    c.extend(Module.new{def bar; end})
+    assert_equal([:bar, :foo], c.singleton_methods.sort - [:yaml_tag])
+    assert_equal([:foo], c.singleton_methods(false))
+  end
+
   def test_singleton_class
     x = Object.new
     xs = class << x; self; end