[FIXED] MQTT PUBREL header incompatibility

[levb]

https://hivemq.github.io/mqtt-cli/docs/test/ pointed out the incompatibility.

[levb]

@wallyqs Please advice where can I add running this test, mqtt test -V 3 -h {NATS_SERVER}

[derekcollison]

LGTM

[wallyqs]

@levb I'm thinking that maybe you can add a test under mqtt_test.go that starts the server and checks whether mqtt is installed and if not installed it skips the test, then in README-MQTT add the instructions about the dependency needed to run the compatibility test and we use a different worker for test (not sure about adding installing mqtt-cli as part of the travis run since it is Java).

[levb]

@wallyqs please check the new test/action

[wallyqs]

LGTM