when empty, members.delete raises ActiveFedora::ObjectNotFoundError "Can't reload an object that hasn't been saved"

[elrayle]

If members aggregation is empty and members.delete is called, it raises ActiveFedora::ObjectNotFoundError "Can't reload an object that hasn't been saved". Expected behavior is to return nil.

See related issue #34

[tpendragon]

@elrayle I think https://github.com/projecthydra-labs/activefedora-aggregation/blob/master/spec/activefedora/association_spec.rb#L33 confirms this is fixed, yes?