pgsql: Fix broken jsonb_set() logic for replacing array elements.

Fix broken jsonb_set() logic for replacing array elements.

Commit 0b62fd036 did a fairly sloppy job of refactoring setPath()
to support jsonb_insert() along with jsonb_set(). In its defense,
though, there was no regression test case exercising the case of
replacing an existing element in a jsonb array.

Per bug #14366 from Peng Sun. Back-patch to 9.6 where bug was introduced.

Report: <20161012065349(dot)1412(dot)47858(at)wrigleys(dot)postgresql(dot)org>

Branch
------
REL9_6_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/0e9e64c07ed93fb2be4583adaea28205d3188a0e

Modified Files
--------------
src/backend/utils/adt/jsonfuncs.c | 12 ++++++------
src/test/regress/expected/jsonb.out | 6 ++++++
src/test/regress/sql/jsonb.sql | 1 +
3 files changed, 13 insertions(+), 6 deletions(-)