Added optional parameter "addEmptyObject" to keep model consistent

[stolzda]

The problem when reverse populating a model is that the returned object hierarchy is not consistent.

Example:
Two authors:
{name: "A", _id: 1} , {name: "B", _id: 2}

Two books:
{name: "book1", author: 1}, {name: "book2", author: 1}

Reverse population of authors with books gives you:

[ { name: "A", _id: 1, books: [{name: "book1", author: 1}, {name: "book2", author: 1}] },
{ name: "B", _id: 2} ]

With the new optional argument "addEmptyObject" set to true it gives you:

[ { name: "A", _id: 1, books: [{name: "book1", author: 1}, {name: "book2", author: 1}] },
{ name: "B", _id: 2, books: [] } ]

I.e., the author object is kept consistent (every author has the field books).