webdav: fix identification of directories for Bitrix Site Manager - #2716

Bitrix Site Manager emits `<D:resourcetype><collection/></D:resourcetype>`
missing the namespace on the `collection` tag.  This causes the item
to be identified as a file instead of a directory.

To work around this look at the Microsoft extension prop
`iscollection` which seems to be emitted as well.

backend/webdav/webdav.go

@@ -172,6 +172,11 @@ func itemIsDir(item *api.Response) bool {
 		}
 		fs.Debugf(nil, "Unknown resource type %q/%q on %q", t.Space, t.Local, item.Props.Name)
 	}
+	// the iscollection prop is a Microsoft extension, but if present it is a reliable indicator
+	// if the above check failed - see #2716
+	if t := item.Props.IsCollection; t != nil {
+		return *t != 0
+	}
 	return false
 }