Display multiple metadata values

In order for documents to have metadata values with multiple options
present, we need to do two things:

- in `abstract_document` map over all the values and find the label for
  each
- in `specialist_documents_helper` turn the array into text that says
  "Item 1 + N others"

This commit also refactors the tests to reflect that tag metadata is now
always in Arrays.

This relies on https://github.com/alphagov/finder-api/pull/32 being merged first.
If we wrap this in `Kernel#Array` then we can be backwards compatible and make deployment less painful:

```ruby
present_values = Array(attrs.fetch(key, [])).select { …
```