Revert pkg/nonwriter changes (#1829)

The DoH work (#1619) made changes to pkg/nonwriter.Writer that in
hindsight were not backwards compatible; it added override for the
LocalAddr() and RemoteAddr(). Instead of rolling back that PR, this PR
reverts those changes and creates a DoHWriter for use in the
https-server.go side of things.

This was only caught in the integration test making this hard to catch,
so we add a upstream_file_test.go that tries (doesn't work yet) to test
this in the unit tests as well. Esp. helpful when 'git bisecting'.

Fixes #1826

test/file_upstream_test.go

@@ -0,0 +1,60 @@
+package test
+
+import (
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+// TODO(miek): this test needs to be fleshed out.
+
+func TestFileUpstream(t *testing.T) {
+	name, rm, err := TempFile(".", `$ORIGIN example.org.
+@	3600 IN	SOA sns.dns.icann.org. noc.dns.icann.org. (
+		2017042745 ; serial
+		7200       ; refresh (2 hours)
+		3600       ; retry (1 hour)
+		1209600    ; expire (2 weeks)
+		3600       ; minimum (1 hour)
+	)
+
+        3600 IN NS a.iana-servers.net.
+	3600 IN NS b.iana-servers.net.
+
+www 3600 IN CNAME   www.example.net.
+`)
+	if err != nil {
+		t.Fatalf("Failed to create zone: %s", err)
+	}
+	defer rm()
+
+	// Corefile with for example without proxy section.
+	corefile := `example.org:0 {
+       file ` + name + ` {
+	       upstream
+	}
+	hosts {
+		10.0.0.1 www.example.net.
+		fallthrough
+	}
+}
+`
+	i, udp, _, err := CoreDNSServerAndPorts(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+	defer i.Stop()
+
+	m := new(dns.Msg)
+	m.SetQuestion("www.example.org.", dns.TypeA)
+	m.SetEdns0(4096, true)
+
+	r, err := dns.Exchange(m, udp)
+	if err != nil {
+		t.Fatalf("Could not exchange msg: %s", err)
+	}
+	if r.Rcode == dns.RcodeServerFailure {
+		t.Fatalf("Rcode should not be dns.RcodeServerFailure")
+	}
+	t.Logf("%s", r)
+}