title: why doesn't Ext.form.field.disable automatically revalidate
body: <pre><code>    var bFoo = foo === "true";
      cmp.setDisabled(!bFoo ); //disabled always passes validation
      cmp.validate();
  </code></pre>
  
  <p>why doesn't Ext 3.3 call validate, or clearInvalid, for us in Ext.form.field.disable() ?</p>
  
  <p>when the field is disabled, the invalid-style incorrectly lingers until the field is next validated. I think it should always validate immediately, or clear the style.</p>
  
  <p>this also happens with enable() on a field with allowBlank=false; the disabled-style goes away, but the invalid-style doesn't appear until the user clicks, which is wrong.</p>