Fabric JS - Text Background Opacity

HTML

<script src="https://rawgit.com/kangax/fabric.js/master/dist/fabric.js"></script>
<canvas id="c" width="400" height="400"></canvas>

CSS

canvas {
    border: 1px solid #999;
}
img {
    display: none;
}

JavaScript

/**
 * fabric.js template for bug reports
 *
 * Please update the name of the jsfiddle (see Fiddle Options).
 * This templates uses latest dev verison of fabric.js (https://rawgithub.com/kangax/fabric.js/master/dist/all.js).
 */

// initialize fabric canvas and assign to global windows object for debug
var canvas = window._canvas = new fabric.Canvas('c');

// Do some initializing stuff
fabric.Object.prototype.set({
    transparentCorners: false,
    cornerColor: 'rgba(102,153,255,0.5)',
    cornerSize: 12,
    padding: 5
});

// ADD YOUR CODE HERE
// clear canvas
canvas.clear();

// add red rectangle
canvas.add(new fabric.Rect({
  width: 100,
  height: 80,
  left: 50,
  top: 50,
  fill: 'rgb(255,0,0)',
  opacity: 1
}));

// add green, half-transparent circle
canvas.add(new fabric.Circle({
  radius: 100,
  left: 50,
  top: 50,
  fill: 'rgb(0,255,0)',
  opacity: 0.5
}));

var text = new fabric.Text('Hello World', {
top:80,
left:80,
fontSize:33,
backgroundColor: '#ffffcc',
borderColor:'#000',
opacity: 0.4,
fontFamily:"helvetica"}).setOpacity(0.4);
   	
canvas.add(text);