Use appendStringInfoString instead of appendBinaryStringInfo where possible

Use appendStringInfoString instead of appendBinaryStringInfo where possible

For the jsonpath output, we don't need to squeeze out every bit of
performance, so instead use a more robust coding style.  There are
similar calls in jsonb.c, which we leave alone here since there is
indeed a performance impact for bulk exports.

Discussion: https://www.postgresql.org/message-id/flat/a0086cfc-ff0f-2827-20fe-52b591d2666c%40enterprisedb.com