ENUM: "IS NULL" check with where()

When I try this:

```php
BookQuery::create()
  ->where('book.type IS NULL')
  ->find();
```

.. I get this InvalidClauseException: 
`A clause must contain a question mark in order to be bound to a value`.

`type` is an ENUM column.

http://sandbox.propelorm.org/a9d21c3