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

core/dnsserver/https.go

@@ -4,8 +4,10 @@ import (
 	"encoding/base64"
 	"fmt"
 	"io/ioutil"
+	"net"
 	"net/http"
 
+	"github.com/coredns/coredns/plugin/pkg/nonwriter"
 	"github.com/miekg/dns"
 )
 
@@ -54,3 +56,19 @@ func base64ToMsg(b64 string) (*dns.Msg, error) {
 }
 
 var b64Enc = base64.RawURLEncoding
+
+// DoHWriter is a nonwriter.Writer that adds more specific LocalAddr and RemoteAddr methods.
+type DoHWriter struct {
+	nonwriter.Writer
+
+	// raddr is the remote's address. This can be optionally set.
+	raddr net.Addr
+	// laddr is our address. This can be optionally set.
+	laddr net.Addr
+}
+
+// RemoteAddr returns the remote address.
+func (d *DoHWriter) RemoteAddr() net.Addr { return d.raddr }
+
+// LocalAddr returns the local address.
+func (d *DoHWriter) LocalAddr() net.Addr { return d.laddr }