gTranslate plugin translation auto-trigger

gTranslate plugin will not translate the Ajax output in some cases for some reason. However it can be triggered if the ajax response content type is changed:

```
isset($_SERVER['HTTP_X_GT_LANG']) &&
!empty(isset($_SERVER['HTTP_X_GT_LANG'])) &&
```

This should be added as a hook to `Utils\Ajax::prepareHeaders()`