6.1.1

CHANGES:

• Leave it up to implementations how to interpret last_modified for
  dictionaries, or even not implement it (then returning Error (`Value_expected _). The previous definition was not well founded when the
  dictionary doesn't contain any values directly, and some implementations
  implements last_modified differently from the description on dictionaries.
  (reported in #41, fixed in #42 by @reynir)
• Leave it up to implemetations how to interpret digest for dictionaries,
  or even not implement it (returning Error (`Value_expected _).
  (reported mirage/ocaml-tar#111, fixed in #43 by @reynir)