Commit 0b62fd03 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>