The sub-types of Intransitive Activity (Arrive, etc) would not satisfy
the vocab.IntransitiveActivityType interface, due to not accounting for
the parent WithoutProperties definition.
This fixes that code generation, so that all Activity subtypes will be
able to be properly converted to vocab.ActivityType or
vocab.IntransitiveActivityType.
Updated the PostOutbox code path to properly handle this distinction
when receiving a C2S IntransitiveActivity.
The IsPublic method will return 'true' if the special public collection
defined in the ActivityPub spec is addressed in the 'to', 'bto' 'cc', or
'bcc' properties of the Object or any of the types extending from them.
Cory Slep