title: Helper object is not updating the target position
body: <p>My scene has a cube and a directional light with your Helper. 
  The cube is placed on X: 300 Y: 100 z: 0 and the light is positioned at X: 300 Y: 600 z: 0. 
  The Target object of light has the same position of the cube. </p>
  
  <p>But the target of the helper is being drawn at (0, 0, 0).</p>
  
  <p>How do I make drawing Helper point to the target light? </p>
  
  <p>This is my code:</p>
  
  <pre><code>var geometry = new THREE.CubeGeometry(200, 200, 200);
  var material = new THREE.MeshPhongMaterial({color: 0x00ff00});
  
  var cube = new THREE.Mesh(geometry, material);
  cube.position.set(300, 100, 0);
  
  scene.add(cube);
  
  var color = 0xffffff;
  var intensity = 1;
  
  var light = new THREE.DirectionalLight( color, intensity );     
  light.position.set(300, 600, 0);
  
  light.target.position.set(300, 100, 0);
  
  scene.add( light );
  
  var helper = new THREE.DirectionalLightHelper( light, 20 );
  scene.add( helper );
  helper.update();
  </code></pre>