Maintain other error messages when setting default

Fixes the case for when `numericality` doesn't have a message, but other options do:

```js
  validations: {
    intenger_attribute: {
      numericality: {
        onlyInteger: true,
        messages: {
          onlyInteger: 'Distancia entre surcos debe ser un número'
        }
      }
    }
  }
```