Update store.js

I am not very familiar with neither Ember nor JS. The change that I am proposing is solving problem in my case. 

When Ember makes a request toward my server (spring mvc) spring returns nested entities as array. The id.type cannot be resolved in my case because the id is undefined - in the incoming JSON i do not provide any type. Moreover the relationship is already properly resolved in my case on this line. Simply using the relationship.type allowed proper retrieval of ids from the id object.