Implement Challenges::Base#[]

[sorah]

Acme::Client::Resources::Challenges.new may return a Hash instead of Challenges::* when it encountered unsupported challenge type.

When an user is iterating authz#challenges, they have to check a class of an element in prior to use.

Checking challenge#challenge_type is sometime useful than checking challenge#class when an user has a support of additional challenge_type which are uncovered in this gem.

(And I failed here... https://github.com/sorah/acmesmith/blob/5845dbadd7e9a1ad2ebd4731304d8f59b111ae68/lib/acmesmith/client.rb#L205 )