golang.org/x/oauth2/jwt: Set kid to KeyID of private key

Set the KeyID hint in the token header. This allows remote servers to
identify the key used to sign the message.

Fixes #18307

Change-Id: Ib95398079833aad6b390650b465d7b09b5f53fda
Reviewed-on: https://go-review.googlesource.com/34320
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>

jwt/jwt.go

@@ -105,7 +105,9 @@ func (js jwtSource) Token() (*oauth2.Token, error) {
 	if t := js.conf.Expires; t > 0 {
 		claimSet.Exp = time.Now().Add(t).Unix()
 	}
-	payload, err := jws.Encode(defaultHeader, claimSet, pk)
+	h := *defaultHeader
+	h.KeyID = js.conf.PrivateKeyID
+	payload, err := jws.Encode(&h, claimSet, pk)
 	if err != nil {
 		return nil, err
 	}