GIN (JSONPath) checks equality only on the first node of each operand

When checking equality with an operand that has additional nodes
("2".double()), GIN only considers the first node ("2"). So $.a ==
"2".integer() looks up a match for the json string "2" instead of the
number 2, and silently drops rows that a sequential scan would have
returned.

```
CREATE TABLE t (j jsonb);
INSERT INTO t VALUES ('{"a": 2}'), ('{"a": "2"}'), ('{"a": 3}');
CREATE INDEX t_ops ON t USING gin (j jsonb_ops);
CREATE INDEX t_path_ops ON t USING gin (j jsonb_path_ops);

SET enable_seqscan = off;
SET enable_indexscan = off; -- leave bitmap scans only
SELECT 'seq', count(*) FROM (SELECT j FROM t OFFSET 0) s WHERE j @@ '$.a ==
"2".double()'; -- 1

DROP INDEX t_path_ops;
SELECT 'jsonb_ops', count(*) FROM t WHERE j @@ '$.a == "2".double()';
-- 0, should be 1
SELECT 'jsonb_ops', count(*) FROM t WHERE j @? '$.a ? (@ == (-2).abs())';
-- 0, should be 1

CREATE INDEX t_path_ops ON t USING gin (j jsonb_path_ops);
DROP INDEX t_ops;
SELECT 'jsonb_path_ops', count(*) FROM t WHERE j @@ '$.a == "2".double()';
-- 0, should be 1
```