Pull request: fix dns cache ttl check

Merge in DNS/adguard-home from 2459-dns-ttl to master

Updates #2459.

Squashed commit of the following:

commit 27e74e30b202ab5163ebdbc2c00622099b11a1ff
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon Dec 21 15:00:46 2020 +0300

    all: log changes

commit e476fa5c4b8fd3896fa401f4dc546a5d937746eb
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon Dec 21 14:55:30 2020 +0300

    dnsforward: fix dns cache ttl check

internal/dnsforward/http.go

@@ -167,11 +167,12 @@ func (req *dnsConfig) checkCacheTTL() bool {
 	if req.CacheMinTTL == nil && req.CacheMaxTTL == nil {
 		return true
 	}
+
 	var min, max uint32
 	if req.CacheMinTTL != nil {
 		min = *req.CacheMinTTL
 	}
-	if req.CacheMaxTTL == nil {
+	if req.CacheMaxTTL != nil {
 		max = *req.CacheMaxTTL
 	}