fix keepttl parsing (#6250)

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/cache/setup.go

@@ -241,6 +241,7 @@ func cacheParse(c *caddy.Controller) (*Cache, error) {
 					return nil, fmt.Errorf("cache type for disable must be %q or %q", Success, Denial)
 				}
 			case "keepttl":
+				args := c.RemainingArgs()
 				if len(args) != 0 {
 					return nil, c.ArgErr()
 				}