title: Nested collection created as array instead of collection
body: <p>I am having some trouble when I create a new model form json, with nested attributes that should be parsed as a collection. Here I show how I get to the error: </p>
  
  <p>(Any help appreciated)</p>
  
  <p>1) I create a user like this</p>
  
  <pre><code>user = new App.Models.User( {"email":"tom@gmail.com","id":1,"user_cities_attributes":[{"city_id":431,"user_action":"to-visit","user_id":1}] }
  </code></pre>
  
  <p>2) and then I check in the console what happened, the collection did not get created. :S</p>
  
  <pre><code>user.get('user_cities')
  &gt; undefined # where is my collection?! :(
  user.get('user_cities_attributes')
  &gt; [Object] # mmm weird...
  </code></pre>
  
  <p>If I try after to add a new model to the collection...</p>
  
  <pre><code>cities = new App.Collections.CitiesCollection()
  cities.fetch()
  
  userCity = new App.Models.UserCity( { user: user, city: cities.first(), user_action: "to-visit" } )
  </code></pre>
  
  <p>now everything seems to be fine... what happened????</p>
  
  <pre><code>user.get('user_cities')
  &gt; Backbone.Collection {length: 20, models: Array[20], _byId: Object, model: function, user: User…}
  user.get('user_cities_attributes')
  undefined
  </code></pre>
  
  <p>MODELS</p>
  
  <pre><code>class App.Models.User extends Backbone.RelationalModel
    paramRoot: 'user'
    urlRoot: 'users'
  
    defaults:
      name: null
  
  class App.Models.UserCity extends Backbone.RelationalModel
    paramRoot: 'user_city'
    urlRoot: 'user_cities'
  
    defaults:
      user_action: null
  
    relations: [{
      type: Backbone.HasOne,
      key: 'user',
      relatedModel: 'App.Models.User',
      collectionType: 'App.Collections.UsersCollection',
      keySource: 'user_id',
      includeInJSON: 'id',
      reverseRelation: {
        key: 'user_cities',
        keySource: 'user_cities_attributes',
        includeInJSON: 'id'
      }
    },
    {
      type: Backbone.HasOne,
      key: 'city',
      relatedModel: 'App.Models.City',
      collectionType: 'App.Collections.CitiesCollection',
      includeInJSON: 'id',
      keySource: 'city_id',
      reverseRelation: {
        key: 'users',
        includeInJSON: 'id'
      } 
    }]
  
  class App.Models.City extends Backbone.RelationalModel
    paramRoot: 'city'
    urlRoot: 'cities'
  
    defaults:
      name: null
  
    relations: [{
      type: Backbone.HasMany,
      key: 'places',
      relatedModel: 'App.Models.Place',
      collectionType: 'App.Collections.PlacesCollection',
      keySource: 'places_attributes',
      # parse: true,
      includeInJSON: 'id',
      autoFetch: true,
      reverseRelation: {
        key: 'city',
        includeInJSON: 'id'
      }
        # 'relatedModel' is automatically set to 'Zoo'; the 'relationType' to 'HasOne'.
    }]
  </code></pre>