ocsp: Expose ResponderID in parsed Response.

ResponderID is unmarshalled into either ResponderName or
ResponderKeyHash depending on which one is present.

Fixes golang/go#14971.

Change-Id: I5ba602753d81835da531321e94898408395068dd
Reviewed-on: https://go-review.googlesource.com/34380
Reviewed-by: Adam Langley <agl@golang.org>