josnschema fails when using optional load_default returns MISSING

[Ragsboss]

We have a custom type like this

class Optional(lt.Optional):
    def __init__(self, *args, **kwargs):
        if 'load_default' in kwargs:
            kwargs.pop('load_default')

        super(Optional, self).__init__(*args, load_default=s.MISSING, **kwargs)

    def load(self, data, context=None, *args, **kwargs):
        if data is s.MISSING:
            return self.load_default(context)
        elif data is None:
            return data
        return super(Optional, self).load(
            self.inner_type.load(data, context=context, *args, **kwargs),
            *args, **kwargs
        )

With that use following code to see the failure

>>> import lollipop.types as lt
>>> from lollipop_jsonschema import json_schema as lollipop2jsonschema
>>> 
>>> lollipop2jsonschema(lt.Optional(s.String()))
OrderedDict([('type', 'string')])
>>> lollipop2jsonschema(Optional(s.String()))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/lollipop_jsonschema/jsonschema.py", line 140, in json_schema
    js['default'] = schema.inner_type.dump(default)
  File "/usr/local/lib/python2.7/dist-packages/lollipop/types.py", line 212, in dump
    self._fail('required')
  File "/usr/local/lib/python2.7/dist-packages/lollipop/errors.py", line 63, in _fail
    raise ValidationError(msg)
lollipop.errors.ValidationError: Invalid data: 'Value is required'