Per RFC 5246, 7.4.1.3:

   cipher_suite
      The single cipher suite selected by the server from the list in
      ClientHello.cipher_suites.  For resumed sessions, this field is
      the value from the state of the session being resumed.

The specifications are not very clearly written about resuming sessions
at the wrong version (i.e. is the TLS 1.0 notion of "session" the same
type as the TLS 1.1 notion of "session"?). But every other
implementation enforces this check and not doing so has some odd
semantics.

Change-Id: I6234708bd02b636c25139d83b0d35381167e5cad
Reviewed-on: https://go-review.googlesource.com/21153Reviewed-by: Adam Langley <agl@golang.org>