example code for http://bbrinck.com/post/543491001/intermediate-rack-middleware-tips

rack-caps.rb

module Rack
  
  class Cap

    def initialize(app, options = {})
      @app = app
    end

    def call(env)
      response = @app.call(env)
      status, headers, body = response
      headers['Content-Length']= body.length.to_s
      [status, headers, body.map {|x| x.upcase+"!"}]
    end
    
  end

end

require 'test/unit'
require 'shoulda'
require 'rack'

class CapTest < Test::Unit::TestCase
  
  should 'capitalize body' do
    inner_app = lambda { |env| [200, {'Content-Type' => 'text/plain'}, ['Hello']] }
    app = Rack::Cap.new(inner_app)
    env = Rack::MockRequest.env_for('/hello')
    status, headers, body = app.call(env)
    assert_equal ['HELLO!'], body
  end

  should 'pass all Lint checks' do
    inner_app = lambda { |env| [200, {'Content-Type' => 'text/plain'}, ['Hello']] }
    app = Rack::Lint.new(Rack::Cap.new(inner_app))
    env = Rack::MockRequest.env_for('/hello')
    app.call(env)
  end
  
end