How to stub Capybara's `page#source`

deviant_art_steps.rb

# First, there is another (simple) way to accomplish the task and I would suggest to never use approach shown below. I mean stub page#source.
#
# Given there are several steps which rely on `page#source`, for instance checking contents of downloaded CSV file.
# And we decided to store several CSV files in the cloud 
# And we still want to test their contents
# Then we test output of file generators
# But we want to reuse and DO NOT TOUCH existing steps checking CSV files contents
# Then we decide to stub page#source

# Simple solution, nothing interesting here, you can skip it.
def check_csv_contents(csv, cucumber_table)
  ...
end

Then /^the CSV file contents should match:$/ do |table|
  csv = CSV.generate { |csv| csv << %w(tom and jerry) } # real CSV generation should be preformed here
  check_csv_contents csv, table
end

Then /^old CSV checking step relied on page#source:$/ do |table|
  check_csv_contents page.source, table
end

# BORING!!!
# B O R I N G ! ! !
# An amused alternative, please never do like that :)

Then /^the CSV file contents should match:$/ do |table|
 csv = CSV.generate { |csv| csv << %w(tom and jerry) } # real CSV generation should be preformed here

  new_source_module = Module.new do
    define_method :new_source do
      csv
    end
  end

  page.instance_eval do
    self.class.send :include, new_source_module
    # we need to keep an eye on the original "page#source" to release our stub in the future
    self.class.send :alias_method, :old_source, :source unless respond_to?(:old_source)
    self.class.send :alias_method, :source, :new_source
  end
end

Then /^crazy extra step to unstub page#source$/ do
  page.instance_eval do
    self.class.send :alias_method, :source, :old_source
  end
end