Hangman game with simple class

hangman.rb

require_relative 'hangman_class'

hangman = Hangman.new

hangman.start

until hangman.finish
  puts hangman.answer
  print "#{hangman.limit} Enter your best guess: "
  hangman.guess(gets)
end

hangman.say_good_bye

hangman_class.rb

class Hangman
  attr_reader :finish, :answer, :limit

  def initialize
    @words = %w{spiderman superman powerpuf hangman}
    @finish = false
    @limit = 8
    @win = false
  end  

  def start
    @word = @words[rand(@words.length-1)]
    @answer = "_" * @word.length
  end

  def guess(user_answer)
    @limit -=  1
    user_answer = user_answer[0..-2]
    if user_answer == 'exit' || limit == 0
      @finish = true
      return 
    end 
    if user_answer.length == 1
      index = 0
      @word.each_char do |w|
        if w == user_answer
          @answer[index] = user_answer
        end
        index += 1
      end
    end
    if @answer == @word || user_answer == @word
      @finish = true
      @win = true
    end
  end

  def say_good_bye
    if @win
      puts "You are the winner"
    else
      puts "Bye loser, the answer is #{@word}" 
    end
  end
end

test_hangman_class.rb

require_relative 'hangman_class'
require 'test/unit'

class TestHangman < Test::Unit::TestCase
  def test_startup_value
    Hangman.class_eval do 
      attr_accessor :words, :finish, :limit, :win
    end
    hangman = Hangman.new
    assert(hangman.words.length > 0, "should have more than one in dictionary")
    assert_equal(hangman.finish, false)
    assert_equal(hangman.limit, 8)
    assert_equal(hangman.win, false)
  end

  def test_exit
    Hangman.class_eval do 
      attr_accessor :word
    end
    hangman = Hangman.new
    hangman.guess("exit")
    assert(hangman.finish, "should exit when user type exit")
  end

  def test_find_one_char
    Hangman.class_eval do 
      attr_accessor :word, :answer
    end    
    hangman = Hangman.new
    hangman.start
    hangman.word =   "powerpuf"
    hangman.answer = "________"
    hangman.guess("x\n")
    assert_equal("________", hangman.answer)
    hangman.guess("o\n")
    assert_equal("_o______", hangman.answer)
    hangman.guess("p\n")
    assert_equal("po___p__", hangman.answer)        
  end

  def test_quick_answer
    Hangman.class_eval do 
      attr_accessor :word, :answer
    end    
    hangman = Hangman.new
    hangman.start
    hangman.word =   "powerpuf"
    hangman.answer = "________"
    hangman.guess("powerpuf\n")
  end
end