Always parse radio facets as Arrays

In order to link back to the pre filtered Finders, we need to have a started way of linking backing to select and radio facets. To do this, we turn the radio facet values into an array and take the first value when we check it against the allowed_values. This also means we should remove the test which was safeguarding against this.
This is a slightly clearer approach, and has the added advantage of taking the first value that's allowed, rather than only considering the first value in the array:

```ruby
Array(@value).find { |value|
  allowed_values.map(&:value).include?(value)
}
```

The only problem with it is that the word `value` appears 5 times. Finding better variable names is left as an exercise for the reader.