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

plugin/pkg/nonwriter/nonwriter.go

@@ -2,8 +2,6 @@
 package nonwriter
 
 import (
-	"net"
-
 	"github.com/miekg/dns"
 )
 
@@ -11,11 +9,6 @@ import (
 type Writer struct {
 	dns.ResponseWriter
 	Msg *dns.Msg
-
-	// 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
 }
 
 // New makes and returns a new NonWriter.
@@ -26,9 +19,3 @@ func (w *Writer) WriteMsg(res *dns.Msg) error {
 	w.Msg = res
 	return nil
 }
-
-// RemoteAddr returns the remote address.
-func (w *Writer) RemoteAddr() net.Addr { return w.Raddr }
-
-// LocalAddr returns the local address.
-func (w *Writer) LocalAddr() net.Addr { return w.Laddr }