Cover core interface contracts.

[samuel-williams-shopify]

Summary

• Cover the application builder yield form.
• Cover default application paths, QUERY requests, and HTML responses.
• Use Protocol::HTTP::Header::Cookie to parse request cookies.

Tests

• bundle exec sus test/utopia/request.rb test/utopia/session.rb
• bundle exec sus test/utopia/application.rb test/utopia/middleware.rb test/utopia/request.rb test/utopia/response.rb
• bundle exec rubocop lib/utopia/request.rb test/utopia/application.rb test/utopia/middleware.rb test/utopia/request.rb test/utopia/response.rb