title: fabric.js - get New Height after redimensioning
body: <p>I create group with: </p>
  
  <pre><code>var text    = new fabric.Text(textValue,{left: 20, top: 30, fontSize:25});
  var rect    = new fabric.Rect({
                      width       : text.get('width') + 40,
                      fill        : "#FFFFFF",
                      height      : 100,
                      stroke      : '#000000',
                      strokeWidth : 1
  
                    });
  
      var myGroupObj = new fabric.CustomGroup([  rect,text ], {
        left      : 0,
        top       : 0,
      });
  </code></pre>
  
  <p>Now whene I manually resizes my group and I want to get a new heigth.</p>
  
  <pre><code>myGroupObj.get('height')
  </code></pre>
  
  <p>I have always last one (100).
  Somme idea to get new height, please? </p>