AssociationProxy fields don't have a property value

If you are using `sqlalchemy.ext.associationproxy.association_proxy` for any of your columns, then flask-restless will crash when trying to save. It is checking the properties on fields in the model to find out of they are date/datetime fields, but `AssociationProxy` fields don't have `property` attributes, so that code just crashes:

    File "flask_restless/views.py", line 1381, in put
      return self.patch(instid)
    File "flask_restless/views.py", line 1346, in patch
      data = self._strings_to_dates(data)
    File "flask_restless/views.py", line 880, in _strings_to_dates
      if _is_date_field(self.model, fieldname) and value is not None:
    File "flask_restless/views.py", line 119, in _is_date_field
      prop = getattr(model, fieldname).property

This patch changes it to pass through to the real field on the remote table when an `AssociationProxy` is encountered, so that field can be checked for whether or not this is a date value.