delegate.rb: don't look for methods on Kernel

Instead, look for instance methods of Kernel.
Otherwise, instance methods of Module (which are methods of Kernel
itself) are mistakenly believed to exist, and it fails when calling
Kernel.instance_method().

Closes: https://github.com/ruby/ruby/pull/1422

lib/delegate.rb

@@ -81,7 +81,7 @@ class Delegator < BasicObject
 
     if r && target.respond_to?(m)
       target.__send__(m, *args, &block)
-    elsif ::Kernel.respond_to?(m, true)
+    elsif ::Kernel.method_defined?(m) || ::Kernel.private_method_defined?(m)
       ::Kernel.instance_method(m).bind(self).(*args, &block)
     else
       super(m, *args, &block)

test/test_delegate.rb

@@ -258,4 +258,11 @@ class TestDelegateClass < Test::Unit::TestCase
   def test_dir_in_delegator_class
     assert_equal(__dir__, Bug9403::DC.dir_name, Bug9403::Name)
   end
+
+  def test_module_methods_vs_kernel_methods
+    delegate = SimpleDelegator.new(Object.new)
+    assert_raise(NoMethodError) do
+      delegate.constants
+    end
+  end
 end