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

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

Fixes #933

middleware/pkg/nonwriter/nonwriter_test.go

@@ -0,0 +1,19 @@
+package nonwriter
+
+import (
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestNonWriter(t *testing.T) {
+	nw := New(nil)
+	m := new(dns.Msg)
+	m.SetQuestion("example.org.", dns.TypeA)
+	if err := nw.WriteMsg(m); err != nil {
+		t.Errorf("Got error when writing to nonwriter: %s", err)
+	}
+	if x := nw.Msg.Question[0].Name; x != "example.org." {
+		t.Errorf("Expacted 'example.org.' got %q:", x)
+	}
+}