Rack::Test Issue: Body shouldn't be a string

app.rb

class App
  def call(env)
    [200, {'Content-Type' => 'text/plain'}, "Hello, World!"]
  end
end

config.ru

require './app'
run App.new

test.rb

require './app'
require 'minitest/autorun'
require 'rack/test'

class AppTest < MiniTest::Test
  include Rack::Test::Methods

  def app
    App.new
  end

  def test_body
    get '/'
    assert_equal "Hello, World!", last_response.body
  end
end