I misinterpreted http://tools.ietf.org/html/rfc4880#section-5.5.3
and implemented the sum of 16-bit values, rather than the 16-bit sum
of 8-bit values.

Thanks to Szabolcs Nagy for pointing it out.

R=bradfitz, r, rsc
CC=golang-dev
https://golang.org/cl/5372091