chore: unify with other plugins

subcommands/upgrade

@@ -15,7 +15,7 @@ service-upgrade-cmd() {
   #F -R|--restart-apps "true", whether to force an app restart
   declare desc="upgrade service <service> to the specified versions"
   local cmd="$PLUGIN_COMMAND_PREFIX:upgrade" argv=("$@"); [[ ${argv[0]} == "$cmd" ]] && shift 1
-  declare SERVICE="$1" UPGRADE_FLAGS_LIST="${@:2}"
+  declare SERVICE="$1" UPGRADE_FLAGS_LIST=("${@:2}")
 
   [[ -z "$SERVICE" ]] && dokku_log_fail "Please specify a valid name for the service"
   verify_service_name "$SERVICE"