In has_many nested form, give new elements same classes than existing.

Hello,

When using `has_many` nested form with `new_record: true`, the new nested forms added when clicking on "Add New [Model]" button only have HTML classes `inputs has_many_fields` while existing ones also have classes passed in `has_many` method's `options` parameter.
This patch fixes it and gives all nested forms, new and existing ones, all the classes.

Thanks in advance for reviewing this patch.