middleware/cache: cache 0 will be capped at 5 (#408)

* middleware/cache: cache 0 will be capped at 5

cache 0 would return TTL=0 records, up that to the documented minimum of
5 seconds.

* middleware/cache: check for 0 TTL

Handle 0 TTL differently and return an error, we might need to
special case this in the future.

middleware/cache/cache.go

@@ -125,7 +125,8 @@ func (c *ResponseWriter) Write(buf []byte) (int, error) {
 const (
 	maxTTL  = 1 * time.Hour
 	maxNTTL = 30 * time.Minute
-	minTTL  = 5 * time.Second
+
+	minTTL = 5 // seconds
 
 	defaultCap = 10000 // default capacity of the cache.