DATAREST-340 - AssociationOmittingSerializerModifier now keeps non-persistent property writers.

AssociationOmittingSerializerModifier previously dropped the writer fro a particular property if it couldn't find a PersistentProperty for the bean property. We now keep those writers to make sure that non-persistent, additional fields are written as well.

Related issues: https://github.com/spring-projects/spring-boot/issues/1190