encoding/json: ignore unexported fields in Unmarshal

Go 1.0 behavior was to create an UnmarshalFieldError when a json value name matched an unexported field name. This error will no longer be created and the field will be skipped instead.

Fixes issue 4660.

R=adg, rsc, bradfitz
CC=golang-dev
https://codereview.appspot.com/7139049

Committer: Russ Cox <rsc@golang.org>