class ActionDispatch::Flash::FlashHash

def self.from_session_value(value) #:nodoc:
  case value
  when FlashHash # Rails 3.1, 3.2
    flashes = value.instance_variable_get(:@flashes)
    if discard = value.instance_variable_get(:@used)
      flashes.except!(*discard)
    end
    new(flashes, flashes.keys)
  when Hash # Rails 4.0
    flashes = value['flashes']
    if discard = value['discard']
      flashes.except!(*discard)
    end
    new(flashes, flashes.keys)
  else
    new
  end
end