Hyrax::VisibilityPropagator does not expect AllinsonFlex objects

[laritakr]

see scientist-softserv/utk-hyku#136

When visibility is assigned to an AllinsonFlex object, the visibility does not propagate down to its members. AllinsonFlex type objects are not expected as source objects, so they fall into the NullVisibilityPropogator and throw an error due to a bug in that subclass. Only Hyrax::WorkBehavior and Hyrax::Resource are expected source type classes, and with AllinsonFlex, we are most likely getting an AllinsonFlex source object.

(Note that this only affects imports through Bulkrax, so it may be a Bulkrax issue or an issue related to using them together, rather than only AllinsonFlex.)