Clarify that as:object can also be an as:Offer in case of accept/reject

[mielvds]

At 5.3. AS2 object, the spec says
For as:object, one of the core [Object Types](https://www.w3.org/TR/activitystreams-core/#object-types) MUST be expressed. In addition to these [Object Types](https://www.w3.org/TR/activitystreams-core/#object-types), types ([[URI]](https://www.eventnotifications.net/#biblio-uri)s) originating from vocabularies chosen by an application domain MAY be specified.

However, this is not true for as:Accept and as:Reject where the object must be an as:Offer

[paulwalk]

This raises the more general question: should the specification determine the JSON type that is acceptable - e.g. in the example most as:object will use an array type for its type property, whereas as:Accept and as:Reject use a string (as:Offer) for the type property.

Over at COAR Notify, the implementers are getting confused by this. They expect a specification to determine types (in the sense of "data types" rather than "semantic types"). But, as we discussed a year ago, Linked Data is looser than that, and perhaps the implementers need to programme more defensively, to expect different data types to be used in different circumstances...?