Currently, autocert.Manager always generates EC-based certificates. This change adds an optional field forcing the Manager to use RSA instead. An alternative idea, a "double" certificate, where the Manager presents either RSA or EC certificate based on client's compatibility, doesn't seem to be worth the implementation time given the constant increase in Elliptic Curve cryptography. Fixes golang/go#17744 Change-Id: Idc68abfc698bcff4aad99715baefc06f8fae50ad Reviewed-on: https://go-review.googlesource.com/34570 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>