Fixes failing tests

There test were failing due to the following lines:

```ruby
  refs = fnga.references({})
  test.assert_equal( true, refs.has_key?("A") )
```

This was failing because the test was trying to assert that the [FnGetAtt](https://github.com/howech/cfndsl/blob/master/lib/cfndsl/JSONable.rb#L30) function contained refs which it didn't have. This is because there is no third parameter passed to the underlying [Fn](https://github.com/howech/cfndsl/blob/master/lib/cfndsl/JSONable.rb#L151) class.