This is a split of https://go-review.googlesource.com/23970 (patch set 8) to address only Client changes: 1. Expose expected server name value of TLS SNI ClientHello message 2. Fix a bug where returned error value was nil. Change-Id: I21f571652e9bbef80a2222dc34fce767270b7c48 Reviewed-on: https://go-review.googlesource.com/26852 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>