Enum variants are serialized as strings even when FieldPathMode == Index

```rust
let x = QueryResult::Test(1);
let y = QueryResult::Test(2);

let diff = serde_diff::Config::new()
.with_field_path_mode(FieldPathMode::Index)
.serializable_diff(&x, &y);
let z = ron::to_string(&diff).unwrap();
println!("{}", z);
```

Expected output:
```
[Enter(EnumVariantIndex(1)),Enter(FieldIndex(0)),Value(2),Exit]
```

Actual output:
```
[Enter(EnumVariant("Test")),Enter(FieldIndex(0)),Value(2),Exit]
```