title: Backbone model - collection property not empty on new model creation
body: <p>I have a model with a backbone collection as an attribute. When I create a new model and save it along with its collection to the database it all works fine. </p>
  
  <p>However, when I create a new model, its collection contains the previous models values. All the other properties are blank as expected. </p>
  
  <p>I've defined my model defaults like this: </p>
  
  <pre><code>defaults: {
          InsertionOrderNumber: null,
          ClientID: null,
          CampaignName: null,
          FromDate: null,
          ToDate: null,
          TotalBudget: null,
          ManagementFee: null,
          AgencyCommission: null,
          SourceDocuments: new Uploads() //this is the collection
      }
  </code></pre>
  
  <p>For some reason it seems like the same instance of the collection is being used for all new models of that type. </p>
  
  <p>Any idea why this is happening?</p>
  
  <p>Thanks in advance =)</p>