title: ExtJs ViewModel Bind Field doesn't work on array
body: <p>I'm trying to bind (two ways bind) some field of my form with data provided by my view model.
  When I bind a simple string, it works perfectly. But when I try to bind a form field to an array field inside my viewmodel, modifications are not considered by the view model.</p>
  
  <p>This is my model</p>
  
  <pre><code>Ext.define('Front.model.User', {
      extend : 'Ext.data.Model',
      fields : [{
          name : 'id',
          type : 'int'
      }, {
          name : 'name',
          type : 'string'
      }, {
          name : 'email',
          type : 'string'
      }, {
          name : 'last_connection',
          type : 'date',
          dateFormat : 'Y-m-d H:i:s'
      }, {
          name : 'birth',
          type : 'date',
          dateFormat : 'Y-m-d H:i:s'
      }, {
          name : 'schedule',
          type : 'auto'
      }]
  });
  </code></pre>
  
  <p>This is my ViewModel</p>
  
  <pre><code>Ext.define('Front.view.user.SheetModel', {
      extend : 'Ext.app.ViewModel',
      alias : 'viewmodel.sheet',
      requires : ['Front.model.User'],
      data : {
          currentUser:null
      },
      model : 'Front.model.User'
  });
  </code></pre>
  
  <p>This is my Form</p>
  
  <pre><code>Ext.define('Front.view.user.Sheet', {
      extend : 'Ext.form.Panel',
      requires : ['Front.view.user.SheetController'],
      controller : 'sheet',
      viewModel : {
          type : 'sheet'
      },
      config : {
          currentUser : null
      }, {
          xtype : 'textfield',
          fieldLabel : 'De',
          name : 'morningstarttest',
          width : 60,
          bind : {
              value : '{currentUser.schedule.lundi.am.start}'
          }
      }
  });
  </code></pre>
  
  <p>The bind displays the correct value the first time but doesn't update if I change the value.</p>
  
  <p>The 'schedule' field in a model is an array.</p>
  
  <p>Any ideas how I could solve this problem?</p>