acme: add StatusValid to expected values for Authorize

Let's Encrypt may respond with the "valid" value in status
field to authorization requests:
https://github.com/letsencrypt/boulder/blob/0543691d9e2cbcc1b94baf3b9fab4e64ca2750ef/ra/ra.go#L373-L402

The spec isn't clear on this:
https://tools.ietf.org/html/draft-ietf-acme-acme-02#section-6.4

Either way, this change follows common sense and should not produce
unexpected results.

Change-Id: Iac55aa1fb17d458f6d62e8da875ed86e3e4afbc3
Reviewed-on: https://go-review.googlesource.com/27432
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
3 files changed