Fixes #2185 (closed).
The template app/views/fields/has_one/_show.html.erb wasn't using the correct i18n key to translate the field names of the associated record.
This PR includes a heavy revamp of spec/administrate/views/fields/has_one/_show_spec.rb, which needed some TLC in order to work with it.
The diff for lib/administrate/field/associative.rb looks a bit misleading. The actual change is that I moved the definition of associated_class_name to be above the private declaration.