Defaults that could be assimilated to None were ignored.

Reported by Yari Podio Casares:

We've found a problem when using default values in schema definitions; in
particular, when the default value can be assimilated with 'None' (0, False,
'').

If you try this schema:

test = {
    'schema' : {
        'name': {
            'type' : 'string',
            'required' : True,
            'default' : '',
        },
        'number': {
            'type' : 'integer',
            'required' : True,
            'default' : 0,
        },
        'yesno': {
            'type' : 'boolean',
            'required' : True,
            'default' : False,
        },
    },
}

...you can see that the three fields act as if the hadn't a 'default' value
defined (you cannot do a POST without speciffying a value for each field).