[stylesSet] register only if isn't already registered.

When I use multiple fields ckeditor in the same form, it gives me an error in js:

    Uncaught [CKEDITOR.resourceManager.add] The resource name "default" is already registered. 

config.yml:
```
ivory_ck_editor:
    default_config: admin_config
    configs:
        admin_config:
            styles: default
    styles:
        default:
            - { name: "Blue Title", element: "h2", styles: { color: "Blue" }}
```