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: <20161012065349.1412.47858@wrigleys.postgresql.org>

Branch
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REL9_6_STABLE

Details
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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(-)