Dynamic app static serving of index.html files

## Prerequisites

* [x] I am running the latest version. (`up upgrade`)
* [x] I searched to see if the issue already exists.
* [x] I inspected the verbose debug output with the `-v, --verbose` flag.
* [ ] Are you an Up Pro subscriber?

## Description

I have a node.js app with a `public` directory containing index.html - however visiting `/` does not serve index.html. All the other files in my `public` directory are served fine.

My up.json looks like this:

```
{
"name": "test-app",
"static": {
"dir": "public"
}
}
```

If I add `"type": "static"` to my config, index.html is served fine