title: fetch() method send the default values instead the new ones
body: <p>I have an issue with a model, i instanciate it up with some default values, i am using the <code>set()</code> method to change it, and when i use the <code>fetch()</code> méthod, i get the defaults values instead the new ones, i tried to log <code>myModel.toJSON()</code> in the console and i see the new values, i launch <code>myModel.fetch()</code> i get the defaults values on the server side, crazy no ?</p>
  
  <p>Here is the model prototype:</p>
  
  <pre><code>window.User = Backbone.Model.extend({
      defaults : {
          id : "1",
          username : "anonymous",
          facebook_id : "1235486421",
          facebook_token : "AEZH93FZFEFSFSFS4545154sfsfSF"
      },
      urlRoot: 'http:/localhost:3000/user',
      initialize : function() {
          console.log('User Constructor...');
          this.url = "http://localhost:3000/user/"+this.get('username')+'/'+this.get('facebook_id')+'/'+this.get('facebook_token');
      }
  });
  </code></pre>
  
  <p>Here is the chunck of code where i use <code>fetch()</code>:</p>
  
  <pre><code>    fetch_user: function(){
      var CurrentUser = new User();
      FB.login(function(response) {
          if (response.authResponse) {
              var access_token = response.authResponse.accessToken;
              FB.api('/me', function(response) {
                  console.log('Good to see you, ' + response.name + ', id: '+response.id+', token: '+access_token);
                  CurrentUser.set('username', response.name);
                  CurrentUser.set('facebook_id', response.id);
                  CurrentUser.set('facebook_token', access_token);
                  console.log('Current user is:  ' + CurrentUser.toJSON());
                  CurrentUser.fetch({
                      success: function(_model){
                          console.log('User fetched successfully');
                      },
                      error: function(_model, error){
                          console.log('User fetched unsuccessfully');
                      }
                  });
              });
          }
          else {
              console.log('User cancelled login or did not fully authorize.');
          }
      }, {scope: 'email'});
  },
  </code></pre>
  
  <p>Thanks !</p>