Change ladder.save() to ladder.update() function.

Optimizing database memory. Don't need to create a new ladder when all is changing is a few numbers, and we don't care about the old standings.
Fix ladder processing code update function.

The first implementation had two problems.
1) It wasn't converting the created ladder model from the JSON data into an object. The .toObject() function corrected this.
2) In the model.update() function itself, {upsert:true} was required as a parameter.
Add test for updating ladder model.