class ActionDispatch::Flash::FlashHash

def to_session_value # :nodoc:

:nodoc:
If there are none to keep, returns +nil+.
Builds a hash containing the flashes to keep for the next request. 
def to_session_value # :nodoc:
  flashes_to_keep = @flashes.except(*@discard)
  return nil if flashes_to_keep.empty?
  { "discard" => [], "flashes" => flashes_to_keep }
end