Writing tests for rake tasks

Now, when you are prepared to write the tests, it's time to figure out how to do it. To demonstrate this, assume that we have to write a rake task named send_email, which has these optional arguments: subject and body. The task should write the given strings to a sent_email.txt file (this is for the purpose of simplicity to demonstrate the tests; in reality, you may want to use a real mailer).

Start to solve the problem statement. The following is a basic class Mailer that will be used in the rake task (place this code in the mailer.rb file):

class Mailer
  DESTINATION = 'sent_email.txt'.freeze
  DEFAULT_SUBJECT = 'Greeting!'
  DEFAULT_BODY = 'Hello!'

  def initialize(options = {})
    @subject = options[:subject] || DEFAULT_SUBJECT
    @body = options[:body] || DEFAULT_BODY
  end

  def send
    puts 'Sending email...'
    File.open(DESTINATION, 'w') do |f|
      f << "Subject: #{@subject}\n"
      f << @body
    end
    puts 'Done!'
  end
end

The...