Infer class from record in UniquenessValidator

I've been using neo4j in a Rails app and have wanted to use 

```ruby
 class Person
  include Neo4j::ActiveNode

  property :username, type: String
  validates_uniqueness_of :username
end
```

I've set up the following spec:

```ruby
describe Person do
  it 'validates uniqueness of username' do
      original = create(:person)
      expect(build(:person, username: original.username))
        .to have(1).errors_on(:username)
  end
end
```

Which fails with the following output: