defaults for descriptor enum

NOTE: proto3 enforces enums starting at 0, however proto2 does not, so we have to decide if this is necessary or not.

`google/protobuf/descriptor`

```
- label: 1,
- type: 1,
+ label: 0,
+ type: 0,
```

```
}

- if (message.label !== 1) {
+ if (message.label !== 0) {
writer.uint32(32).int32(message.label);
}

- if (message.type !== 1) {
+ if (message.type !== 0) {
writer.uint32(40).int32(message.type);
}
```

```js
name: isSet(object.name) ? String(object.name) : "",
number: isSet(object.number) ? Number(object.number) : 0,
- label: isSet(object.label) ? fieldDescriptorProto_LabelFromJSON(object.label) : 1,
- type: isSet(object.type) ? fieldDescriptorProto_TypeFromJSON(object.type) : 1,
+ label: isSet(object.label) ? fieldDescriptorProto_LabelFromJSON(object.label) : 0,
+ type: isSet(object.type) ? fieldDescriptorProto_TypeFromJSON(object.type) : 0,
typeName: isSet(object.typeName) ? String(object.typeName) : "",
```