Fix default value check when setting SASL authentication from URI

Author: vilius-g

connection.go

@@ -206,7 +206,22 @@ func DialConfig(url string, config Config) (*Connection, error) {
 	}
 
 	if config.SASL == nil {
-		config.SASL = []Authentication{uri.PlainAuth()}
+		if uri.AuthMechanism != nil {
+			for _, identifier := range uri.AuthMechanism {
+				switch strings.ToUpper(identifier) {
+				case "PLAIN":
+					config.SASL = append(config.SASL, uri.PlainAuth())
+				case "AMQPLAIN":
+					config.SASL = append(config.SASL, uri.AMQPlainAuth())
+				case "EXTERNAL":
+					config.SASL = append(config.SASL, &ExternalAuth{})
+				default:
+					return nil, fmt.Errorf("unsupported auth_mechanism: %v", identifier)
+				}
+			}
+		} else {
+			config.SASL = []Authentication{uri.PlainAuth()}
+		}
 	}
 
 	if config.Vhost == "" {
@@ -226,21 +241,6 @@ func DialConfig(url string, config Config) (*Connection, error) {
 		connectionTimeout = time.Duration(uri.ConnectionTimeout) * time.Millisecond
 	}
 
-	if config.SASL == nil && uri.AuthMechanism != nil {
-		for _, identifier := range uri.AuthMechanism {
-			switch strings.ToUpper(identifier) {
-			case "PLAIN":
-				config.SASL = append(config.SASL, uri.PlainAuth())
-			case "AMQPLAIN":
-				config.SASL = append(config.SASL, uri.AMQPlainAuth())
-			case "EXTERNAL":
-				config.SASL = append(config.SASL, &ExternalAuth{})
-			default:
-				return nil, fmt.Errorf("unsupported auth_mechanism: %v", identifier)
-			}
-		}
-	}
-
 	addr := net.JoinHostPort(uri.Host, strconv.FormatInt(int64(uri.Port), 10))
 
 	dialer := config.Dial