json: unmarshal types that are byte slices.

The json package cheerfully would marshal

        type S struct {
                IP net.IP
        }

but would give an error when unmarshalling.  This change allows any
type whose concrete type is a byte slice to be unmarshalled from a
string.

Fixes #5086.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/11161044

src/pkg/encoding/json/decode.go

@@ -660,7 +660,7 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool
 		default:
 			d.saveError(&UnmarshalTypeError{"string", v.Type()})
 		case reflect.Slice:
-			if v.Type() != byteSliceType {
+			if v.Type().Elem().Kind() != reflect.Uint8 {
 				d.saveError(&UnmarshalTypeError{"string", v.Type()})
 				break
 			}