Correctly handle vhost in connection urls.

It is possible to pass the url of a RabbitMQ server in the format

ampq://user:pass@hostname:port/vhost

However the leading slash for the vhost was mistakenly stripped which caused
urls that included a path not to work properly. This patch fixes the issue.

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I briefly looked into writing a test for this but I think we'd need to expose more from `lib/connect.js` to achieve this.