Cleanups and tests (#272)

For some reasons there was a dnsserver/middleware.go that defined
the middleware handlers. This code was a repeat from
middleware/middleware.go. Removed dnsserver/middleware.go and replaced
all uses of dnsserver.Middleware with middleware.Middleware.

Added dnsserver/address_test.go to test the zone normalization (and to
improve the test coverage). The deleted file will also improve the test
coverage :)

core/dnsserver/address_test.go

@@ -0,0 +1,28 @@
+package dnsserver
+
+import "testing"
+
+func TestNormalizeZone(t *testing.T) {
+	for i, test := range []struct {
+		input     string
+		expected  string
+		shouldErr bool
+	}{
+		{".", ".:53", false},
+		{".:54", ".:54", false},
+		{"..", ":", true},
+		{"..", ":", true},
+	} {
+		addr, err := normalizeZone(test.input)
+		actual := addr.String()
+		if test.shouldErr && err == nil {
+			t.Errorf("Test %d: Expected error, but there wasn't any", i)
+		}
+		if !test.shouldErr && err != nil {
+			t.Errorf("Test %d: Expected no error, but there was one: %v", i, err)
+		}
+		if actual != test.expected {
+			t.Errorf("Test %d: Expected %s but got %s", i, test.expected, actual)
+		}
+	}
+}