Update backbone.js

When a collection fetches updates on its models it uses the _prepareModel method to set up a new model with the fetched attributes. Either this model becomes a new model in the collection or it is used to merge with an existing model.

The problem is that when you instantiate a new model the initialize method runs. This method is often customized by the developer and can contain some harmful, or at least totally unnecessary, code for merging purposes. 

Specifically I run into a scenario when my model class listened to events in the initialize method, causing the temporary model to never stop listening after the merge.

But this is just one scenario. I think it is a good principle to not trigger the initializer since Backbone tries to do its own thing and triggering initilize can cause all sorts of havoc.

Instead the initialize method is run right before it is added to the collection, so the only delay is the check for "merge" or "add".