title: Overriding defaultHeaders and method not working Ext.Ajax override with sencha 6.2
body: <p>I had Ext.Ajax overriden with defaultHeaders and method in Extjs4.2-</p>
  
  <pre><code>Ext.define('overrides.Ajax', {
  override: 'Ext.Ajax',
  defaultHeaders: { 'Content-Type': 'application/json; charset=utf-8' },
  method: 'POST',
  </code></pre>
  
  <p>But with Extjs 6.2 defaultHeaders and method is not setting. Any idea on the issue?</p>