pgsql: Teach jsonpath string() to unwrap in lax mode

Teach jsonpath string() to unwrap in lax mode

This was an ommission in commit 66ea94e, and brings it into compliance
with both other methods and the standard.

Per complaint from David Wheeler.

Author: David Wheeler, Jeevan Chalke
Reviewed-by: Chapman Flack

Discussion: https://postgr.es/m/A64AE04F-4410-42B7-A141-7A7349260F4D@justatheory.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/653d3969bb013f14c4a6884a253ad9676caf8166

Modified Files
--------------
doc/src/sgml/func.sgml                       |  5 ++++-
src/backend/utils/adt/jsonpath_exec.c        |  3 +++
src/test/regress/expected/jsonb_jsonpath.out | 12 +++++++++++-
src/test/regress/sql/jsonb_jsonpath.sql      |  1 +
4 files changed, 19 insertions(+), 2 deletions(-)