One file Rails 4 app (https://github.com/rails/rails/issues/17045)

test.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', '4.1.6'
    gem 'rack'
    gem 'pg', '0.17.1'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'rails'
require 'action_controller/railtie'

require 'yaml'

class TestApp < Rails::Application
  config.root = File.dirname(__FILE__)
  config.session_store :cookie_store, key: 'cookie_store_key'
  config.secret_token    = 'secret_token'
  config.secret_key_base = 'secret_key_base'

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

  routes.draw do
    get '/' => 'test#test'
  end
end

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

  def test
    render text: 'hello!'
  end
end

require 'minitest/autorun'
require 'rack/test'

class BugTest < Minitest::Test
  include Rack::Test::Methods

  POST_DATA = {
    "proposal" => {
      "title" => "foo",
      # "cover_photo" => Rack::Test::UploadedFile.new(File.open(File.join(Rails.root, 'Gemfile'))),
      "profile_sections" => [
        {"title" => "d", "content" => "e"},
        {"title" => "f", "content" => "g"}
      ]
    }
  }

  JSON_DATA = '{"some_scalar_attribute":"foo","some_array_of_scalars":["b","c"],"some_array_of_hashes_containing_scalars":[{"scalar_one":"d","scalar_two":"e"},{"scalar_one":"f","scalar_two":"g"}]}'

  def test_returns_success
    post '/', POST_DATA
    assert last_response.ok?
    assert_equal JSON_DATA, last_response.body
  end

  private
    def app
      Rails.application
    end
end