Honor documented default: (and default_response:) in 2.3; stop reading route.desc #991

Parent tracker: #987.

Two leftover reader-migration items we can do in grape-swagger 2.3 without waiting on Grape.

1. Documented default: is ignored

The README "Default response" section documents:

desc 'thing', default: { message: 'the default response' }

The code has only ever read route.default_response. So the documented spelling is a silent no-op, and the spelling that works is undocumented (only in specs).

2.3: read both:

route.try(:default_response) || route.settings.dig(:description, :default)

• try(:default_response) keeps the working (undocumented) spelling without depending on delegate_missing_to forever (try is nil when respond_to? is false).
• route.default looks like the matching Grape reader, but it is Hash#default: BaseRoute def_delegators :default onto @options, which is ActiveSupport::OrderedOptions < Hash. It always returns nil. Do not use it. route.settings is a real reader; :description already holds the Hash-form and block-form default: payload.
• Do not go back to route.options[:…].

Document default_response: as the preferred form; note that default: still works until 3.0. grape-swagger 3.0 can drop the default: fallback once Grape adds default_response as a real desc key and aliases default onto it (see #987).

2. Stop reading route.desc

desc 'text' stores :description. route.desc is almost never set and is one of the four method_missing reads in #987. Drop summary = route.desc if route.desc in summary_object; keep route.description + route.detail + route.summary. No Grape reader for desc.

desc 'This returns something' with no detail/summary already puts that text in swagger description (api_swagger_v2_detail_spec.rb, detail: nil example).