Fix fromJson method.

[ChristianGiupponi]

JWT plugins returns data in "data" key.

Example from Postman:

{
    "success": true,
    "statusCode": 200,
    "code": "jwt_auth_valid_credential",
    "message": "Credential is valid",
    "data": {
        "token": "token string here",
        "id": 1,
        "email": "info@example.com",
        "nicename": "christian",
        "firstName": "Christian",
        "lastName": "Giupponi",
        "displayName": "christian"
    }
}