instance_method owners don't always respond to method_defined?

barator.rb

module Barator
  def foo(*args)
    "#{super(*args)}bar"
  end
end

class Foo
  prepend Barator

  def foo
    'foo'
  end
  def to_s
    foo
  end
end

describe Foo do
  specify { expect(Foo.new.foo).to eq 'foobar' }

  [:foo, :to_s].each do |method_name|
    context "##{method_name}" do
      it "has an owner that responds to method_defined?" do
        expect(Foo.instance_method(method_name).owner).to respond_to(:method_defined?)
      end

      it "is stubbable" do
        expect { Foo.new.stub(method_name) }.to_not raise_error
      end
    end
  end
end

output.txt

Foo ...FF
  1) Foo#to_s has an owner that responds to method_defined?
     Failure/Error: expect(Foo.instance_method(method_name).owner).to respond_to(:method_defined?)
       expected #<Foo:0x007f960d393118> to respond to :method_defined?
     # ./spec/models/barator_spec.rb:24:in `block (4 levels) in <top (required)>'
  2) Foo#to_s is stubbable
     Failure/Error: expect { Foo.new.stub(method_name) }.to_not raise_error
       expected no Exception, got #<NoMethodError: undefined method `method_defined?' for #<Foo:0x007f960d393118>> with backtrace:
         # ./spec/models/barator_spec.rb:28:in `block (5 levels) in <top (required)>'
         # ./spec/models/barator_spec.rb:28:in `block (4 levels) in <top (required)>'
     # ./spec/models/barator_spec.rb:28:in `block (4 levels) in <top (required)>'

Finished in 0.00682 seconds
5 examples, 2 failures

Failed examples:

rspec ./spec/models/barator_spec.rb:23 # Foo#to_s has an owner that responds to method_defined?
rspec ./spec/models/barator_spec.rb:27 # Foo#to_s is stubbable

Randomized with seed 573