def _fetch_strategy(name, scope)
      @strategies[scope][name] ||= if klass = Warden::Strategies[name]
        klass.new(@env, scope)
      elsif @config.silence_missing_strategies?
        nil
      else
        raise "Invalid strategy #{name}"
      end
    end
def _fetch_strategy(name, scope)
      @strategies[scope][name] ||= if klass = Warden::Strategies[name]
        klass.new(@env, scope)
      elsif @config.silence_missing_strategies?
        nil
      else
        raise "Invalid strategy #{name}"
      end
    end
Fetches strategies and keep them in a hash cache.