Fix copy-paste error in datum_to_jsonb_internal()

Fix copy-paste error in datum_to_jsonb_internal()

Commit 3c152a27b06 mistakenly repeated JSONTYPE_JSON in a condition,
omitting JSONTYPE_CAST. As a result, datum_to_jsonb_internal() failed
to reject inputs that were casts (e.g., from an enum to json as in the
example below) when used as keys in JSON constructors.

This led to a crash in cases like:

  SELECT JSON_OBJECT('happy'::mood: '123'::jsonb);

where 'happy'::mood is implicitly cast to json. The missing check
meant such casted values weren’t properly rejected as invalid
(non-scalar) JSON keys.

Reported-by: Maciek Sakrejda <maciek@pganalyze.com>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Maciek Sakrejda <maciek@pganalyze.com>
Discussion: https://postgr.es/m/CADXhmgTJtJZK9A3Na_ry+Xrq-ghjcejBRhcRMzWZvbd__QdgJA@mail.gmail.com
Backpatch-through: 17