Do not output properties when they are undefined

Basically this let us have for the same price
JSON.stringify(john) : {"height": 4}
instead of {"height": 4, "name": null}

Weirdly it wasn't easy to achieve.
This requires https://github.com/dresende/node-sql-query/pull/37 for it to pass tests.

Mind that i tested only postgres (and sqlite ? i don't know) but kept in mind it should work everywhere.