middleware/whoami: add (#264)

Add a new middleware that tells you who you are; IP, port and transport
is echoed back.

Also some various cleanup and documentation improvements while at it:

* ResponseWriter: improve the documentation of these helper functions.
* And add an NextHandler for use in tests. Make chaos_test.go and
* whoam_test.go use it.

middleware/whoami/whoami_test.go

@@ -0,0 +1,62 @@
+package whoami
+
+import (
+	"testing"
+
+	"github.com/miekg/coredns/middleware"
+	"github.com/miekg/coredns/middleware/pkg/dnsrecorder"
+	"github.com/miekg/coredns/middleware/test"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+func TestWhoami(t *testing.T) {
+	wh := Whoami{}
+
+	tests := []struct {
+		next          middleware.Handler
+		qname         string
+		qtype         uint16
+		expectedCode  int
+		expectedReply []string // ownernames for the records in the additional section.
+		expectedErr   error
+	}{
+		{
+			next:          test.NextHandler(dns.RcodeSuccess, nil),
+			qname:         "example.org",
+			qtype:         dns.TypeA,
+			expectedCode:  dns.RcodeSuccess,
+			expectedReply: []string{"example.org.", "_udp.example.org."},
+			expectedErr:   nil,
+		},
+	}
+
+	ctx := context.TODO()
+
+	for i, tc := range tests {
+		wh.Next = tc.next
+		req := new(dns.Msg)
+		req.SetQuestion(dns.Fqdn(tc.qname), tc.qtype)
+
+		rec := dnsrecorder.New(&test.ResponseWriter{})
+		code, err := wh.ServeDNS(ctx, rec, req)
+
+		t.Logf("%s\n", rec.Msg)
+
+		if err != tc.expectedErr {
+			t.Errorf("Test %d: Expected error %v, but got %v", i, tc.expectedErr, err)
+		}
+		if code != int(tc.expectedCode) {
+			t.Errorf("Test %d: Expected status code %d, but got %d", i, tc.expectedCode, code)
+		}
+		if len(tc.expectedReply) != 0 {
+			for i, expected := range tc.expectedReply {
+				actual := rec.Msg.Extra[i].Header().Name
+				if actual != expected {
+					t.Errorf("Test %d: Expected answer %s, but got %s", i, expected, actual)
+				}
+			}
+		}
+	}
+}