Can't use inherited relationships in includes

[andrus]

When doing a GET on a superclass in an inheritance hierarchy, Agrest allows to include attributes from any subclass, but when including a relationship, an exception is thrown:

i.a.j.p.JaxrsAgExceptionMapper: 500 Internal Server Error (Exception processing Agrest request) 
[cause: No such relationship: BaseEntity.relName]

This happens under very specific condition:

• Inherited entity is itself a child entity of another relationship
• There is an "exp" in the request