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

tv42/old-go,apparentlymart/golang,tv42/old-go,tv42/old-go,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,gnanderson/go,gnanderson/go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,apparentlymart/golang,gnanderson/go,gnanderson/go,tv42/old-go,gnanderson/go,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,tv42/old-go