[PATCH] fix GIN index search sometimes losing results

It appeared than GIN index sometimes lose results if simultaneously:

1 if query operand contains weight marks 

2 if weight-marked operand is negated by ! operator

3 if there are only logical (not phrase) operators from this negation towards the root of query tree. 

e.g. '!crew:A'::tsquery refuse to find 'crew:BCD'::tsvector

Seems it is in all versions of PG.

The patch is intended to deal with the issue. Also it contains tests for these rare condition.

Pavel Borisov.