Commit f1d669ae added support for
AES_256_GCM_SHA384 cipher suites as specified in RFC5289. However, it
did not take the arbitrary hash function into account in the TLS client
handshake when using client certificates.

The hashForClientCertificate method always returned SHA256 as its
hashing function, even if it actually used a different one to calculate
its digest. Setting up the connection would eventually fail with the
error "tls: failed to sign handshake with client certificate:
crypto/rsa: input must be hashed message".

Included is an additional test for this specific situation that uses the
SHA384 hash.

Fixes #9808

Change-Id: Iccbf4ab225633471ef897907c208ad31f92855a3
Reviewed-on: https://go-review.googlesource.com/7040Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>