Clarification of String and Array behavior for plan exports

[pengyin-shan]

Found this in the middle of rubocop fix:
Running rspec ./spec/features/plans/exports_spec.rb after fixing the export_plan.rb are seeing following issue:

May need to discuss this with the main code team, since it seems that we are using different data properties to define some export setting

[pengyin-shan]

Fixed by change the line 4 of _plan_text.erb to <%= Array(@hash[:attribution]).many? ? _("Creators: ") + Array(@hash[:attribution]).join(", ") : _('Creator:') + @hash[:attribution] %>. The commit will be part of #264 PR. Close this issue as completed.