activesupport_to_json_bug.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"
  # Activate the gem you are reporting the issue against.
  gem "mail", '2.6.4'
  gem "activesupport", "4.2.7"
end

require 'mail'
require "active_support/all"
require "minitest/autorun"

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

class BugTest < Minitest::Test
  def test_stuff
    mail = Mail.new do
      to	  'example@example.org'
      from    'example@example.net'
      subject 'to_json with (JPG) attachment'
    end
    mail.add_file("example.jpg")


    assert mail.to_json.is_a?(String) # Crash and burn
    assert_equal Encoding::UTF_8, mail.to_json.encoding
  end
end

json_works.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"
  # Activate the gem you are reporting the issue against.
  gem "mail", '2.6.4'
  gem "json", "1.8.3"
  gem 'minitest'
end

require 'mail'
require "json"
require "minitest/autorun"

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

class BugTest < Minitest::Test
  def test_stuff
    mail = Mail.new do
      to          'example@example.org'
      from    'example@example.net'
      subject 'to_json with (JPG) attachment'
    end
    mail.add_file("example.jpg")


    assert mail.to_json.is_a?(String) # Crash and burn
    assert_equal Encoding::UTF_8, mail.to_json.encoding
  end
end