ensure WebDAVProvider.rootPath is a folder

[sdumetz]

Setting up the default WebDAV server, if you create a folder (cube in my example) with a dummy scene with it and navigate to http://localhost:8000/voyager-story-dev.html?document=/cube/scene.svx.json WITHOUT setting the root parameter, the scene kinda works but for the asset tree not matching properly.

Easiest way to show this is click Create in the Article Task. Nothing is shown until you click somewhere to deselect said article.

I traced it back to WebDAVProvider.rootPath, being normally set to /cube/ but defaults to the value of ExplorerApplication.props.document, here /cube/scene.svx.json.

I checked usage of this path and it looks like it's only used in WebDAVProvider.parseResponse

Not sure if expecting rootPath to always end with a trailing "/" is a valid option though? It doesn't seem to work otherwise anyway? if I set root=/cube the scene won't load