Pull request: all: fix client upstreams, imp code

Updates #3186.

Squashed commit of the following:

commit a8dd0e2cda3039839d069fe71a5bd0f9635ec064
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri May 28 12:54:07 2021 +0300

    all: imp code, names

commit 98f86c21ae23b665095075feb4a59dcfcc622bc7
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu May 27 21:11:37 2021 +0300

    all: fix client upstreams, imp code

internal/dnsforward/dns_test.go

@@ -379,3 +379,18 @@ func TestServer_ProcessLocalPTR_usingResolvers(t *testing.T) {
 		require.Empty(t, proxyCtx.Res.Answer)
 	})
 }
+
+func TestIPStringFromAddr(t *testing.T) {
+	t.Run("not_nil", func(t *testing.T) {
+		addr := net.UDPAddr{
+			IP:   net.ParseIP("1:2:3::4"),
+			Port: 12345,
+			Zone: "eth0",
+		}
+		assert.Equal(t, ipStringFromAddr(&addr), addr.IP.String())
+	})
+
+	t.Run("nil", func(t *testing.T) {
+		assert.Empty(t, ipStringFromAddr(nil))
+	})
+}