Fix node-redis warning

node_redis: Deprecated: The SETEX command contains a argument of type Promise.
This is converted to "[object Promise]" by using .toString() now and will return an error from v.3.0 on.
Please handle this in your code to make sure everything works as you intended it to.

lib/middleware/redis-cache.js

@@ -46,7 +46,7 @@ module.exports = function(options = {}) {
             if (key) {
                 const value = await redisClient.get(key);
                 if (value) {
-                    let ttl = redisClient.ttl(key);
+                    let ttl = await redisClient.ttl(key);
                     ttl = ttl > expire ? ttl : expire * 2;
                     await redisClient.setex(key, ttl, value);
                 }