`App.use(path, ...)` Does Not Mount at `'//'`

`Router` instance mounted explicitly at `'//'` exposes routes when path begins only with a single `/`:

```javascript
const TestRoutes = express.Router();

TestRoutes.get('/drop/table/:table', (req, res) => {
// drop table
res.send(`Dropped table '${req.params.table}'`);
})

TestRoutes.get('/', sendListOfTestRoutes);

if (TEST) app.use('//', TestRoutes);

app.listen();
```

Expected behavior is that navigating to `http://hostname/` would bypass this `Router` and go to the main app, but instead Express serves the list of test routes mounted within the router at `//`.