No data for NOT expressions with non existing value and index

[kuba--]

CREATE INDEX email_idx ON commits USING pilosalib (commit_author_email);
SELECT count(*) FROM commits WHERE  NOT (commit_author_email='non existing trash');

Get 0 rows. It doesn't work for both drivers (pilosa and pilosalib).

[kuba--]

Also expression like:

SELECT count(*) FROM commits WHERE (commit_author_email='existing email') AND (commit_author_email!='non existing email');

doesn't work

[kuba--]

Fix: #326