can't stringify a record if it contains mutable properties and no primary key

[asherf]

If a primary key is not defined and there is a mutable property.
(For example a property defined with isa=dict) calling str(record) will fail:
KeyHashError: PK for returned an unhashable value; try setting a primary key in type MyRecord

[samv]

Looks like __hash__ is broken for those values, too