Prevent empty structs if no attributes, relationships or fetched properties exist
Fix conflicting return type by making subentity's objectID class a subclass of the superentity's objectID class
Check if there is a superentity, fix broken test
Merge pull request #150 from danielctull-forks/master

[FIX] Prevent empty structs if no attributes, relationships or fetched properties exist. Issue #133. (Daniel Tull)