acme: add support for ECDSA-based signatures

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>
2 files changed