Arrayify some input to DocumentFilter::Filterer

Strings get passed through to the document filter from query parameters
like "world_locations=all". Arrayifying means we can be sure we're
dealing with an array and call #any? on it.
Merge pull request #1040 from alphagov/fix-announcements-atom-500-error

Arrayify some input to DocumentFilter::Filterer

https://www.pivotaltracker.com/story/show/59416132