format_reset.rb

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"
  gem "rails", github: "rails/rails"
  gem "arel", github: "rails/arel"
end

require "action_controller/railtie"

class TestApp < Rails::Application
  config.root = File.dirname(__FILE__)
  secrets.secret_token    = "secret_token"
  secrets.secret_key_base = "secret_key_base"

  config.logger = Logger.new($stdout)
  Rails.logger  = config.logger

  routes.draw do
    get "/" => "test#type"
  end
end

class TestController < ActionController::Base
  include Rails.application.routes.url_helpers

  def type
    Rails.logger.info "PATH_INFO=#{request.env['PATH_INFO'].inspect}, format=#{request.format.to_s}"
    render plain: request.format
  end
end

require "minitest/autorun"
require "rack/test"

class BugTest < ActionController::TestCase
  setup do
    @controller = TestController.new
    @routes = ActionDispatch::Routing::RouteSet.new.tap do |r|
      r.draw do
        ActiveSupport::Deprecation.silence do
          get ':controller(/:action(/:id))'
        end
      end
    end
  end

  def test_missing_format_defaults_back_to_html
    # uncomment to make the test pass
    #get :type

    get :type, format: 'json'
    assert_equal 'application/json', @response.body

    get :type
    assert_equal 'text/html', @response.body
  end
end