Allow setInstanceProperty to accept undefined and return true in order t...

...o facilitate https://github.com/dresende/node-sql-query/commit/8ecf962eeeb4aa244ec1159ca75d603ce99212ba

https://github.com/dresende/node-orm2/issues/391
https://github.com/dresende/node-sql-query/issues/29

This change is required to allow a default of undefined to be passed through to the sql-generator.