Change RawToken.Decode signature

Passing a Header address as first argument doesn't look natural, and
header information is not always used. Forcing the user to unmarshal a
header structure is counterintuitive and therefore it is now an optional
value returned by RawToken.Decode.

benchmark_test.go

@@ -52,11 +52,8 @@ func BenchmarkVerify(b *testing.B) {
 		if err = raw.Verify(hs256); err != nil {
 			b.Fatal(err)
 		}
-		var (
-			h Header
-			p benchPayload
-		)
-		if err = raw.Decode(&h, &p); err != nil {
+		var p benchPayload
+		if _, err = raw.Decode(&p); err != nil {
 			b.Fatal(err)
 		}
 	}

jwt_test.go

@@ -93,7 +93,7 @@ func testJWT(t *testing.T, testCases []testCase) {
 				h2  Header
 				tp2 testPayload
 			)
-			err = raw.Decode(&h2, &tp2)
+			h2, err = raw.Decode(&tp2)
 			if want, got := tc.decodingErr, err; want != got {
 				t.Errorf("want %v, got %v", want, got)
 			}

raw_token.go

@@ -39,33 +39,32 @@ func Parse(token []byte) (RawToken, error) {
 }
 
 // Decode decodes a raw JWT into a header and a payload.
-func (r RawToken) Decode(h *Header, payload interface{}) error {
+func (r RawToken) Decode(payload interface{}) (h Header, err error) {
 	// Next, unmarshal the token accordingly.
 	var (
 		enc      []byte // encoded header/payload
 		dec      []byte // decoded header/payload
 		encoding = base64.RawURLEncoding
-		err      error
 	)
 	// Header.
 	enc = r.header()
 	dec = make([]byte, encoding.DecodedLen(len(enc)))
 	if _, err = encoding.Decode(dec, enc); err != nil {
-		return err
+		return
 	}
-	if err = json.Unmarshal(dec, h); err != nil {
-		return err
+	if err = json.Unmarshal(dec, &h); err != nil {
+		return
 	}
 	// Claims.
 	enc = r.claims()
 	dec = make([]byte, encoding.DecodedLen(len(enc)))
 	if _, err = encoding.Decode(dec, enc); err != nil {
-		return err
+		return
 	}
 	if err = json.Unmarshal(dec, payload); err != nil {
-		return err
+		return
 	}
-	return nil
+	return
 }
 
 // Verify verifies a JWT signature with a verifying method that