Class detector

include/jwt-cpp/jwt.h:2344–2347

		// https://en.cppreference.com/w/cpp/experimental/is_detected
		template<class Default, class AlwaysVoid, template<class...> class Op, class... Args>
		struct detector {
			using value = std::false_type;
			using type = Default;
		};

		template<class Default, template<class...> class Op, class... Args>
		struct detector<Default, void_t<Op<Args...>>, Op, Args...> {