gob: decode into nil, this time for sure.

Yesterday's change was too simple-minded and failed if an
interface value was being discarded.  We need to parse the
data stream and remember any type information that arrives.

Also fix a minor bug when ignoring an interface: toss only what
we know about, not everything.

R=rsc
CC=golang-dev
https://golang.org/cl/4179045
3 files changed