function setStrProp(src, dst, propName, def) {
        if (src[propName] && typeof src[propName] == "string") {
            dst[propName] = src[propName];
        } else {
            if(def != undefined) dst[propName] = def;
        }
    }
Helper function for setting string property values in the MQTT V5 properties object
@param {object} src Source object containing properties
@param {object} dst Destination object to set/add properties
@param {string} propName The property name to set in the Destination object
@param {string} [def] An optional default to set in the destination object if prop is NOT present in the soruce object