title: Cannot override method afterRequest on Ext.data.proxy.Ajax instance when migrating an Ext JS 4 codebase
body: <p>Background: I want to be able to create a custom error mask for ajax loading errors, for ex. in case of network problems.</p>
  
  <p>I am using the following code to create to create a a new Ext JS store, whose proxy I would like to extend with my own <code>afterRequest</code> function.
  The <code>afterRequest</code> function should send an <code>afterload</code> event, which is used by a  utility function calling <code>mask()</code> on the widget.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>var settingsStore = new Ext.data.Store({
    model: 'vmSettings',
    autoload: 'true',
    proxy: {
      type: 'ajax',
      url: 'ta-debian-7-wheezy-virtualbox.json',
      afterRequest: function(request, success) {
        this.fireEvent('afterload', this, request, success);
        return;
      },
      reader: {
        type: 'json',
        rootProperty: 'variables',
      },
    }
  
  });</code></pre>
  </div>
  </div>
  </p>
  
  <p>Now this code is loading withour error with Ext JS but fails on 5.1 with the message:  <strong>Cannot override method afterRequest on Ext.data.proxy.Ajax instance</strong> </p>
  
  <p>What could be wrong here ? </p>