Maintain other error messages when setting default

Fixes the case for when `numericality` doesn't have a message, but other options do:

TODO: Test Drive It.

```js
var Controller = Ember.ObjectController.extend(Ember.Validations.Mixin, {
  validations: {
    row_spacing_cm: {
      numericality: {
        onlyInteger: true,
        messages: {
          onlyInteger: 'Distancia entre surcos debe ser un número'
        }
      }
    }
  }
});
```