module Sinatra::Helpers

http://tools.ietf.org/html/rfc2616#section-14.9.1
See RFC 2616 / 14.9 for more on standard cache control directives:

=> Cache-Control: public, must-revalidate, max-age=60
cache_control :public, :must_revalidate, :max_age => 60

a Hash of value directives (:max_age, :min_stale, :s_max_age).
:no_store, :must_revalidate, :proxy_revalidate) may be passed along with
Any number of non-value directives (:public, :private, :no_cache,
Specify response freshness policy for HTTP caches (Cache-Control header).

def cache_control(*values)
  if values.last.kind_of?(Hash)
    hash = values.pop
    hash.reject! { |k,v| v == false }
    hash.reject! { |k,v| values << k if v == true }
  else
    hash = {}
  end
  values = values.map { |value| value.to_s.tr('_','-') }
  hash.each do |key, value|
    key = key.to_s.tr('_', '-')
    value = value.to_i if key == "max-age"
    values << [key, value].join('=')
  end
  response['Cache-Control'] = values.join(', ') if values.any?
end