Merging rule loader overwrites existing

[eliperelman]

"module": {
        "rule": {
          "css": {
            "loader": {
              "css": {
                "options": {
                  "modules": true
                }
              }
            }
          }
        }
      }
    }

This should extend the rule.css.loader.css if one exists, but currently it overwrites.