Move nonwriter to mw/pkg/nonwriter (#948)

Make it its own package as shared between autopath and federation.

Fixes #933

middleware/autopath/autopath.go

@@ -36,6 +36,7 @@ import (
 
 	"github.com/coredns/coredns/middleware"
 	"github.com/coredns/coredns/middleware/pkg/dnsutil"
+	"github.com/coredns/coredns/middleware/pkg/nonwriter"
 	"github.com/coredns/coredns/request"
 
 	"github.com/miekg/dns"
@@ -101,7 +102,7 @@ func (a *AutoPath) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Ms
 	for i, s := range searchpath {
 		newQName := base + "." + s
 		ar.Question[0].Name = newQName
-		nw := NewNonWriter(w)
+		nw := nonwriter.New(w)
 
 		rcode, err := middleware.NextOrFailure(a.Name(), a.Next, ctx, nw, ar)
 		if err != nil {

middleware/autopath/nonwriter.go

@@ -1,22 +0,0 @@
-package autopath
-
-import (
-	"github.com/miekg/dns"
-)
-
-// NonWriter is a type of ResponseWriter that captures the message, but never writes to the client.
-type NonWriter struct {
-	dns.ResponseWriter
-	Msg *dns.Msg
-}
-
-// NewNonWriter makes and returns a new NonWriter.
-func NewNonWriter(w dns.ResponseWriter) *NonWriter { return &NonWriter{ResponseWriter: w} }
-
-// WriteMsg records the message, but doesn't write it itself.
-func (r *NonWriter) WriteMsg(res *dns.Msg) error {
-	r.Msg = res
-	return nil
-}
-
-func (r *NonWriter) Write(buf []byte) (int, error) { return len(buf), nil }