BUG #16146: ts_headline does not find phrase matches correctly

The following bug has been logged on the website:

Bug reference: 16146
Logged by: Augustinas Jokubauskas
Email address: augustinas@spotlightdata.co.uk
PostgreSQL version: 12.0
Operating system: Ubuntu 18.04.3
Description:

When query:
```
select ts_headline(
$$Lorem ipsum urna. Nullam nullam ullamcorper urna.$$,
to_tsquery('Lorem') && phraseto_tsquery('ullamcorper urna'),
'StartSel=#$#, StopSel=#$#, FragmentDelimiter=$#$, MaxFragments=100,
MaxWords=100, MinWords=1'
);
```
is ran, a fragment of

Lorem ipsum urna. Nullam nullam ullamcorper urna.

should be returned, however, the result is a single word of #$#Lorem#$# is
returned, meaning that ts_headline did not find the queried string.