encoding/json: decode [] as empty slice, not nil slice
encoding/json: decode [] as empty slice, not nil slice

Test was already present, but bug in reflect.DeepEqual hid this bug.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5375090
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go