Validate the document_class option.

def validate_document_class(option, value):
    """Validate the document_class option."""
    if not issubclass(value, (collections.MutableMapping, RawBSONDocument)):
        raise TypeError("%s must be dict, bson.son.SON, "
                        "bson.raw_bson.RawBSONDocument, or a "
                        "sublass of collections.MutableMapping" % (option,))
    return value