```ruby
require_relative 'lib/contentful-sync-rss'

module ContentfulSyncRss
  class Proxy < Goliath::API

    use Goliath::Rack::Render
    include Goliath::Rack::Templates

    def on_headers(env, headers)
      if !env['HTTP_CLIENT_ID']
        raise Goliath::Validation::Error.new(
          400, "Required header: Client-Id.")
      end
    end

    def response(env)
      client_id = env['HTTP_CLIENT_ID']

      space = redis.get "clients:#{client_id}:space"
      access_token = redis.get "clients:#{client_id}:access_token"
      next_sync_url = redis.get "clients:#{client_id}:next_sync_url"
    def render_output(format, items)
      case format
      when :rss
        builder(:rss, locals: {items: items})
      else
        raise Errors::UnsupportedFormat
      end
    end

      unless space && access_token
        return [401, {}, "Authentication failed."]
      end

      url = if next_sync_url.nil?
        "https://cdn.contentful.com/spaces/#{space}/sync?initial=true"
      else
        next_sync_url
      end

      items = []
      response = nil
      begin
        content = EM::HttpRequest.new(url).get query: {'access_token' => access_token}
        if content.response_header.status == 200
          logger.info "Received #{content.response_header.status} from Contentful"
          response = JSON.parse(content.response)
          items += response['items']
        else
          raise ::ContentfulSyncRss::Errors::SyncApiError
        end
      end while url = response['nextPageUrl']

      rss = builder(:rss, locals: {items: items})

      if response['nextSyncUrl']
        redis.set("clients:#{client_id}:next_sync_url", response['nextSyncUrl'])
      end

      [200, {}, rss]
    end

  end
end
```

Apply changes to main application (extensible rendering, models, request/response classes)
user
```ruby
require_relative 'lib/contentful-sync-rss'

module ContentfulSyncRss
  class Proxy < Goliath::API

    use Goliath::Rack::Render
    include Goliath::Rack::Templates

    def on_headers(env, headers)
      if !env['HTTP_CLIENT_ID']
        raise Goliath::Validation::Error.new(
          400, "Required header: Client-Id.")
      end
    end

    def render_output(format, items)
      case format
      when :rss
        builder(:rss, locals: {items: items})
      else
        raise Errors::UnsupportedFormat
      end
    end

    def response(env)
      begin
        client = Client.find(redis, env['HTTP_CLIENT_ID'])
      rescue Errors::ClientNotFound
        return [401, {}, "Authentication failed."]
      end

      url = if client.next_sync_url.nil?
        "https://cdn.contentful.com/spaces/#{client.space}/sync?initial=true"
      else
        client.next_sync_url
      end

      response = RequestHandler.request(url, client.access_token)
      output = render_output(:rss, response.items)

      client.next_sync_url = response.next_sync_url
      client.save(redis)

      [200, {}, output]
    end

  end
end
```
assistant