fix connecting on uris with trailing slash and no vhost

This PR fixes the case where this would fail:

```js
amqp.connect('amqp://127.0.0.1:5672/')
```

but this would succeed:
```js
amqp.connect('amqp://127.0.0.1:5672');
```

The trailing slash in the first example becomes an empty vhost as determined in connect.js: https://github.com/hopkinsth/amqp.node/blob/a972f63edf9321c901cf016b3a6f642d72d54380/lib/connect.js#L53