add collection to model attrs during _prepareModel

This matches the [Backbone behavior](https://github.com/jashkenas/backbone/blob/master/backbone.js#L937).

This was causing [a failing test](https://github.com/AmpersandJS/ampersand-collection-rest-mixin/commit/e99422b317932bf88c6a49072b5dbcfe415b72d7) in the rest mixin when doing `create` with `wait:true`. Since that would cause `collection.add(model)` to not be called immediately, the model would never get a reference to the collection.