Failing test when only an Array param present

I ran into an issue today while using `permit_params` like this:

```ruby
permit_params label_ids: Array
```

We only needed to permit an array param. We're running version 0.3 and an `ActiveModel::ForbiddenAttributes` exception is thrown. I wrote a failing test for this (although I don't know if you'd want to include this - the naming is a bit poor here).

This isn't a huge issue for us, but I thought I'd share. We're using a workaround like this instead:

```ruby
permit_params(
  :placeholder,
  label_ids: Array
)
```

Just let me know if a full pull request would be helpful. This might not be worth it as it might be an edge case for most people.