Enum in class is not working: lead.proto.fs(251, 13): [FS0001] No overloads match for method 'OfJson'.

Snippet like the below

```
enum SourceEnum {
FACEBOOK = 0;
OTHER = 1;
}
message Source {
/* Only used if the source_type = OTHER */
string other = 1;
SourceEnum source = 2;
}

```

generates code
```
[]
type Source =
{ mutable other : string option
mutable source : SourceEnum option }

static member JsonObjCodec =
(fun other source ->
{ other = other
source = source }) Operators.jopt ("other") (fun x -> x.other)
<*> Operators.jopt ("source") (fun x -> x.source)
```

which generates error

> lead.proto.fs(251, 13): [FS0001] No overloads match for method 'OfJson'. The available overloads are shown below.

Which seems to be referring to this line
> <*> Operators.jopt ("source") (fun x -> x.source)

as the problem