QWebEngineUrlSchemeHandler, ajax and request return code.

It seems that requests using a custom scheme (and a custom QWebEngineUrlSchemeHandler) have no return status code.

Using the simple sample attached program.

When running it with `QTWEBENGINE_REMOTE_DEBUGGING=9090` and connecting to localhost:9090. We can see that network request are 0B len and there is no request or response header.

While it works for most of the request, it seems that, for ajax requests, the response `status` is never changed and is keep to 0. So the classical ajax test `this.readyState == 4 && this.status == 200` is never true.

If requesting the url `http://localhost:8080/index.html` (and serving the same content using a local server), everything works (ajax response status is set to 200, remote debugging show correct response status code/headers for the requests)