`axlsx-rails` gem bug: `invalid byte sequence in UTF-8`. The views are split into multiple files because that seems to help trigger the bug

_add_title.xlsx.axlsx

sh.add_row ['Your message:']

_first_page.xlsx.axlsx

render partial: '/first_row',
  handlers: [:axlsx],
  formats: [:xlsx],
  layout: false,
  locals: { sh: sh }

sh.add_row [message]

_first_row.xlsx.axlsx

sh.add_row ['This is a test']

render partial: '/add_title',
  handlers: [:axlsx],
  formats: [:xlsx],
  layout: false,
  locals: { sh: sh }

axlsx_invalid_utf8.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 'byebug'
  gem 'rails',
    '4.1.8'
    # '5.0.2'
  gem 'axlsx',
    '2.1.0.pre'
  gem 'axlsx_rails',
    '0.5.0'
end

require 'action_controller/railtie'

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

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

  self.view_paths = '.'

  def index
    render template: 'book.xlsx.axlsx',
      handlers: [:axlsx],
      formats: [:xlsx],
      layout: false,
      locals: { message: params[:message] || 'hi there' }
  end
end

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

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

  def test_returns_success
    get '/', message: 'qwerty'
    assert last_response.ok?

    # keep generated file
    File.open 'amessage.xlsx', 'wb' do |xlsx|
      xlsx << last_response.body
    end
  end

  private

  def app
    Rails.application
  end
end

book.xlsx.axlsx

xlsx_package.workbook do |book|
  book.add_worksheet(name: 'First Page') do |sh|
    sh.sheet_view.show_grid_lines = false

    render partial: '/first_page',
      handlers: [:axlsx],
      formats: [:xlsx],
      layout: false,
      locals: { sh: sh, message: message }
  end
end