Requesting Flashes is always seen as modification

[camille-sound4]

If you use Flashes to get the flash messages at each request (to process templates for instance), the session is always modified, even if there was no flash.
This adds unnecessary cookie updates.

To avoid this, s.written should be set to true only if the Flashes array len is > 0.

func (s *session) Flashes(vars ...string) []interface{} {
	flashes := s.Session().Flashes(vars...)
	s.written = len(flashes) > 0
	return flashes
}