Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/dnstap/handler_test.go

@@ -0,0 +1,65 @@
+package dnstap
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+
+	"github.com/coredns/coredns/plugin/dnstap/test"
+	mwtest "github.com/coredns/coredns/plugin/test"
+
+	tap "github.com/dnstap/golang-dnstap"
+	"github.com/golang/protobuf/proto"
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+func testCase(t *testing.T, tapq, tapr *tap.Message, q, r *dns.Msg) {
+	w := writer{}
+	w.queue = append(w.queue, tapq, tapr)
+	h := Dnstap{
+		Next: mwtest.HandlerFunc(func(_ context.Context,
+			w dns.ResponseWriter, _ *dns.Msg) (int, error) {
+
+			return 0, w.WriteMsg(r)
+		}),
+		Out:  &w,
+		Pack: false,
+	}
+	_, err := h.ServeDNS(context.TODO(), &mwtest.ResponseWriter{}, q)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+type writer struct {
+	queue []*tap.Message
+}
+
+func (w *writer) Write(b []byte) (int, error) {
+	e := tap.Dnstap{}
+	if err := proto.Unmarshal(b, &e); err != nil {
+		return 0, err
+	}
+	if len(w.queue) == 0 {
+		return 0, errors.New("message not expected")
+	}
+	if !test.MsgEqual(w.queue[0], e.Message) {
+		return 0, fmt.Errorf("want: %v, have: %v", w.queue[0], e.Message)
+	}
+	w.queue = w.queue[1:]
+	return len(b), nil
+}
+
+func TestDnstap(t *testing.T) {
+	q := mwtest.Case{Qname: "example.org", Qtype: dns.TypeA}.Msg()
+	r := mwtest.Case{
+		Qname: "example.org.", Qtype: dns.TypeA,
+		Answer: []dns.RR{
+			mwtest.A("example.org. 3600	IN	A 10.0.0.1"),
+		},
+	}.Msg()
+	tapq := test.TestingData().ToClientQuery()
+	tapr := test.TestingData().ToClientResponse()
+	testCase(t, tapq, tapr, q, r)
+}