[9.x] Call prepare() on HttpException responses

[tillkruss]

When the AbstractRouteCollection::handleMatchedRoute() throws a NotFoundHttpException, the generated response is never passed through Router::toResponse() which calls the Response::prepare() method.

The end result of this is that the when a non-existing route is called the response doesn't contain the Content-Type: text/html header and the response may not be rendered correctly, depending on whether the web server will "intelligently" add the Content-Type header.

I ran into this when running Octane using my own AWS Lambda handler that doesn't correct this, like vapor-core does.