$ref to definitions inside “source” or “with” parts of $merge is not supported

Hi,

When the "with" object contains definitions (like '"$ref": "#/definitions/JwtConfig"'), ajv throws an error.

I need to move the "with" "definitions" property to the main object :

```
{
$merge: {
source
with
},
definitions
}
```

I think it's an issue :)

Regards