A previous change, https://go-review.googlesource.com/25462, added support only for encoding JWK with a public ECDSA key. Client users weren't actually able to make any request using a EC key because of invalid JWS header and signature. This change adds EC-based header and signature, which makes the Client to have full support for ECDSA keys now. Change-Id: Ia88f08465ce941f72d2001019f5aa47441a17d6e Reviewed-on: https://go-review.googlesource.com/27438 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>