Need to deal with merging in has_many relations that are initialized to an empty collection.

[malomalo]

The Listing#index.json in MLS doesn't return photos.

These photos get initialized to an empty array when a new model is created.

When Backbone.Collection merges models together it creates an empty array and will overwrite a collection full of data retrieved from Listing#show.

Either we shouldn't initialize the array (maybe lazily initialize a collection on Model.get). Or mark so that when merging in it's discarded. Not sure on the best approach.