Additionally, ActivityPub::FetchRemoteStatusService no longer parses
activities.
OStatus::Activity::Creation no longer delegates to ActivityPub because
the provided ActivityPub representations are not signed while OStatus
representations are.
SerializarbleResource#as_json serializes to Symbol keyed Hash, but current
implementation of LinkedDataSignature expects String keyed Hash.
So it generates broken payload.
* Add handling of Linked Data Signatures in payloads
* Add a way to sign JSON, fix canonicalization of signature options
* Fix signatureValue encoding, send out signed JSON when distributing
* Add missing security context
