#vary allows specifying a default that is already handled

[rynonl]

Ex:

.vary :split_name, context: 'foo' do |v|
  v.when :variant1, :variant2 do
    ...
  end
  v.default :variant1 do
    ...
  end
end

the behavior here is unclear for visitors assigned variant1, it's likely we should not allow the default to be specified in another when.