Handling boolean parameter

Not sure, where to put this, but if you add the following lines to `get_configspace()` in
`example_mlp_on_digits.py` the notebook crashes:

```
boolean = CSH.CategoricalHyperparameter('boolean', [True, False])
config_space.add_hyperparameter(boolean)
```

I noticed that when I was trying to run CAVE on my own data, so I guess having such a parameter is a common use case. Is there anything I can do about this or is it already fixed in a newer version not available via pip?

Start with get_configspace() in example_mlp_on_digits.py and reproduce the notebook crash after adding the boolean CategoricalHyperparameter shown in the issue. Inspect the related configuration and notebook behavior; done means a configuration containing True/False can be added and the example runs without crashing.

Written by the indexing model from the issue text.