Replace the field with the most features with a 'default' field.

def _set_default_key(mapping):
        """Replace the field with the most features with a 'default' field."""
        key_longest = sorted([(len(v), k) for k, v in mapping.items()],
                             reverse=True)[0][1]
        mapping['default'] = key_longest
        del (mapping[key_longest])