title: Fabric.js toJSON() method missing image
body: <p>I am having difficulty returning all canvas objects in the .toJSON() method.</p>
  
  <p>in my test <a href="http://jsfiddle.net/EqeZj/" rel="nofollow">http://jsfiddle.net/EqeZj/</a> (Fabric.js v1.4.8) it seems to only be returning the shape object.</p>
  
  <pre><code>// Do some initializing stuff
      fabric.Object.prototype.set({
          transparentCorners: false,
          cornerColor: 'rgba(102,153,255,0.5)',
          cornerSize: 12,
          padding: 5
      });
  
      // initialize fabric canvas and assign to global windows object for debug
      var canvas = window._canvas = new fabric.Canvas('c');
  
      canvas.add(new fabric.Circle({
          radius: 250,
          left: 10,
          top: 10,
          fill: '#0B61A4'
      }));
  
      fabric.Image.fromURL('http://jsfiddle.net/img/logo.png', function(img) {
        img.left = 58;
        img.top = 110;
        canvas.add(img);
      });
  
      canvas.includeDefaultValues = false;
      fabric.log('JSON without default values: ', canvas.toJSON());
      fabric.log('canvas.getObjects ', canvas.getObjects());
  </code></pre>
  
  <p>Am I doing this correctly?</p>