Support default_response and default in 2.3; stop reading route.desc

Follow-up from #987 for grape-swagger 2.3. Not blocked on Grape.

default: is ignored

The README "Default response" section documents desc ..., default: { ... }. The code has only ever read default_response:. The documented spelling silently produces no default response.

Grape's route.default cannot be the fallback: on released Grape it is OrderedOptions#default, i.e. Hash#default, so it is nil even when the :default key is set. Dual-support without going back to route.options is:

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

try is nil when respond_to? is false, so this still works if Grape drops delegate_missing_to before a real default_response reader exists. settings is a real reader.

default_response: stays the preferred spelling. default: remains until grape-swagger 3.0. Grape HEAD already remaps default: onto default_response (ruby-grape/grape#2862); grape-swagger 3.0 can then read default_response only.

Stop reading route.desc

desc 'text' stores :description. route.desc is almost never set and is not a Grape reader. Drop summary = route.desc if route.desc in summary_object. Keep route.description, route.detail, and route.summary. No Grape reader for desc.

Out of scope

success / failure fallbacks stay until 3.0. No grape-swagger issue for adding a desc reader.

Start with the README's “Default response” section and the summary_object entry point. Trace the existing route metadata handling, then verify that both default_response: and default: produce a default response while summary generation no longer consults route.desc; keep description, detail, and summary behavior intact.