Normalize domains

We should ignore the domains' case, and treat them uniformly, specially when it
comes to local domains.

This patch extends the existing normalization (IDNA, keeping domains as
UTF8 internally) to include case conversion and NFC form for
consistency.

internal/normalize/normalize_test.go

@@ -33,10 +33,42 @@ func TestUser(t *testing.T) {
 	}
 }
 
+func TestDomain(t *testing.T) {
+	valid := []struct{ user, norm string }{
+		{"ÑAndÚ", "ñandú"},
+		{"Pingüino", "pingüino"},
+		{"xn--aca-6ma", "ñaca"},
+		{"xn--lca", "ñ"}, // Punycode is for 'Ñ'.
+		{"e\u0301", "é"}, // Transform to NFC form.
+	}
+	for _, c := range valid {
+		nu, err := Domain(c.user)
+		if nu != c.norm {
+			t.Errorf("%q normalized to %q, expected %q", c.user, nu, c.norm)
+		}
+		if err != nil {
+			t.Errorf("%q error: %v", c.user, err)
+		}
+
+	}
+
+	invalid := []string{"xn---", "xn--xyz-ñ"}
+	for _, u := range invalid {
+		nu, err := Domain(u)
+		if err == nil {
+			t.Errorf("expected Domain(%+q) to fail, but did not", u)
+		}
+		if nu != u {
+			t.Errorf("%+q failed norm, but returned %+q", u, nu)
+		}
+	}
+}
+
 func TestAddr(t *testing.T) {
 	valid := []struct{ user, norm string }{
 		{"ÑAndÚ@pampa", "ñandú@pampa"},
 		{"Pingüino@patagonia", "pingüino@patagonia"},
+		{"pe\u0301@le\u0301a", "pé@léa"}, // Transform to NFC form.
 	}
 	for _, c := range valid {
 		nu, err := Addr(c.user)