members.delete should return values consistent with Array and Hash

[elrayle]

Array and Hash return nil if the item to delete is out of range or the specified key is missing. If the item to delete is correctly deleted, then the deleted item is returned.

Similarly, it is expected that members.delete and related_objects.delete will return...

• return child_collection/object when successful delete
• return nil if child_collection/object does not exist in parent_collection/object
• raise error for any other problems

Related to samvera/hydra-pcdm#103.

[elrayle]

See also #36, which describe similar issues for related_objects.

[elrayle]

No longer appears to be an issue.