call (Rack::ConditionalGet)

    # File lib/rack/conditionalget.rb, line 20
        def call(env)
          return @app.call(env) unless %w[GET HEAD].include?(env['REQUEST_METHOD'])

          status, headers, body = @app.call(env)
          headers = Utils::HeaderHash.new(headers)
          if etag_matches?(env, headers) || modified_since?(env, headers)
            status = 304
            body = []
          end
          [status, headers, body]
        end