cast protobuf enum return 0

version: 1.5  1.3

```
test.proto

enum N {
   DEFAULT = 0;
   ONE =  1;
}

message Test {
    N  num  = 1;
}

```


```
main.go

a := &pb.Test{ Num:  123 }

cast.toInt32(a)   //return 0
int32(a)   //return 123

```
