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 used here in my case because the id.type is undefined - in the incoming JSON i do not provide any type and id contains information from the incoming JSON. 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.