before_update hook

[marcoarthur]

Before update code to be used when expiration happens:

has temperature => (
  traits => ['Cache'],
  is => 'rw',
  isa => 'Int',
  builder => _get_temperature,
  required => 1,
  expiration_time => 10,
  before_update => sub { my ($self, $expired) = @_; $self->save_or_discard($expired) }
);