Document that `UsedSpaceAnnouncement.ToV2` method returns
`nil` when is called on `nil`. Document that `NewAnnouncementFromV2`
method returns `nil` when is called on `nil`. Add
corresponding unit test.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>
Remove usage of deprecated of `container.ID` and `token.SessionToken` code
elements. Replace using of custom message generators with the ones provided
by packages. Replace string comparison with `Equal` method call.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
