String enum members should be passable to decorators expecting strings

The following should work IMO:

```
enum ValidFormats {
ipv4, ipv6
}

@format(ValidFormats.ipv4)
scalar ipv4Address extends string;
```

I think the current error is correct - an enum member isn't assignable to string. But, if the decorator were instead declared using `valueof string` (ala #888), string enum members could be coerced to their string values.