Use username/password from MQTTConfig if not present in uri

[albertov]

Also utf8-encode username as spec mandates

3.1.3.4 User Name
If the User Name Flag is set to 1, this is the next field in the payload. The User Name MUST be a UTF-8 encoded string as > defined in Section 1.5.3 [MQTT-3.1.3-11]. It can be used by the Server for authentication and authorization.

[dustin]

Awesome, thanks!

I couldn't figure out how to get github actions to run the build with the updated actions, so I merged it in locally and tried it, so it went in as 88db37d