ListField raises TypeError on load

[FPurchess]

Traceback (most recent call last):
  [...]
    instance = yield MyDocument.get(id__exact=_id)
  [...]
  File "/local/lib/python2.7/site-packages/motorengine/queryset.py", line 285, in handle
    doc = self.__klass__.from_son(instance)
  File "/local/lib/python2.7/site-packages/motorengine/document.py", line 59, in from_son
    field_values[field.name] = cls._fields[field.name].from_son(value)
  File "/local/lib/python2.7/site-packages/motorengine/fields/list_field.py", line 62, in from_son
    return list(map(self._base_field.from_son, value))
TypeError: argument 2 to map() must support iteration

How to reproduce it:

• store a document
• extend the document with a ListField
• try to load the stored document

[FPurchess]

Bugfix: https://github.com/heynemann/motorengine/pull/71