Renased Ngram

This test seems enough for the feature. However, we still need tests for the other parts.

For instance, we need to guarantee that all terms of the `edge_ngram` are being properly indexed, see [BackendIndexationTestCase](https://github.com/notanumber/xapian-haystack/blob/master/tests/xapian_tests/tests/test_backend.py#L187).