Why do mocked objects not assume identity of the object type they're mocking?

result.txt

$> rspec test_case.rb
F.

Failures:

  1) a failing case expects success
     Failure/Error: expect(production_code(response)).to eq 'success'

       expected: "success"
            got: "failure"

       (compared using ==)
     # ./test_case.rb:28:in `block (2 levels) in <top (required)>'

Finished in 0.00106 seconds (files took 0.35682 seconds to load)
2 examples, 1 failure

Failed examples:

rspec ./test_case.rb:26 # a failing case expects success

test_case.rb

require 'rspec'

module MyRequest
  Good = Class.new
  Redirect = Class.new
  Fail = Class.new
end

def production_code(response)
  case response
  when MyRequest::Good
    'success'
  when MyRequest::Redirect, # 301
      'redirect'
  else
    'failure'
  end
end

RSpec.configure do |config|
  config.mock_with :rspec do |mocks|
    mocks.verify_partial_doubles = true
    mocks.verify_doubled_constant_names = true
  end
end

RSpec.describe "a failing case" do
  it "expects success" do
    response = instance_double('MyRequest::Good')
    expect(production_code(response)).to eq 'success'
  end

  it 'expects failure' do
    response = instance_double('MyRequest::Fail')
    expect(production_code(response)).to eq 'failure'
  end
end

Fixed typo, added requires.