Do not uri escape on default vhost (slash), fixes queue check

[Napsty]

The plugin does a uri_escape on the vhost, which is by default set to /. This then creates a %2F character instead of the / slash character, failing to find the URL:

$ /usr/lib/nagios/plugins/check_rabbitmq_queue.pl -H localhost --port 15672 --username monitoring --password secret
RABBITMQ_QUEUE CRITICAL - Access Refused : http://localhost:15672/api/queues/%2F

With this PR, this is fixed and only does a uri_escape on actual vhost names, not on /.

$ /tmp/check_rabbitmq_queue.pl -H localhost --port 15672 --username monitoring --password secret
RABBITMQ_QUEUE OK - All queues under the thresholds | messages=0.8636;; messages_ready=0.8636;; messages_unacknowledged=0.0000;; consumers=0.5682;;

This probably also fixes #106 .