Correctly display enums consisting of arrays in the show page

The show page displays correctly enums when they are hashes or arrays of
arrays, but it incorrectly displays enums consisting simply of arrays,
such as:
['value', 'other_value']

This is the form used in Rails enums. This patch takes such enums into
account.
Merge remote-tracking branch 'inaka/take_new_rails4_enums_into_account'

* inaka/take_new_rails4_enums_into_account:
  Correctly display enums consisting of arrays in the show page