Fixes #3559.

This makes Marshal handle fields marked ",any" instead of ignoring
them. That makes Marshal more symmetrical with Unmarshal, which seems
to have been a design goal.

Note some test cases were changed, because this patch changes
marshalling behavior. I think the previous behavior was buggy, but
there's still a backward-compatibility question to consider.

R=rsc
CC=golang-dev, n13m3y3r
https://golang.org/cl/6938068