不把文件流载到内存，怎样直接通过RestClient下载一个大文件

require 'rest-client'
File.open('/tmp/README.md', 'w') {|f|
  block = proc { |response|
    response.read_body do |chunk|
      #puts "Working on response"
      f.write chunk
    end
  }

  RestClient::Request.new(method: :get, url: 'https://raw.githubusercontent.com/rails/rails/master/README.md', block_response: block).execute  
}