It got out of sync after the round of API design changes, this brings it
back in line.
Also fix the behavior of passing nil as the config to match the
expected/documented behavior.
Change-Id: I8b51f5bd29ef9338ddb920fb872d857f180f9c84
Reviewed-on: https://go-review.googlesource.com/127255
Reviewed-by: Michael Matloob <matloob@golang.org>