BUG #13533: jsonb_populate_record does not work when the value is a simple string

The following bug has been logged on the website:

Bug reference: 13533
Logged by: Paulo Vieira
Email address: paulovieira(at)gmail(dot)com
PostgreSQL version: 9.5alpha1
Operating system: Linux Ubuntu 14.4
Description:

1) Reproduce the problem:

---------------------------------------------------
drop table if exists temp_table;
create table temp_table(id int, data json);

do $$

declare
input_data json := '{"id": 1, "data": "abc"}';
input_row temp_table%ROWTYPE;

begin
for input_row in (select * from
json_populate_record(null::temp_table,input_data)) loop
raise notice '%', input_row.data;
end loop;
end

$$;
---------------------------------------------------

2) Output I got:

ERROR: 22P02: invalid input syntax for type json
DETAIL: Token "abc" is invalid.
CONTEXT: JSON data, line 1: abc
PL/pgSQL function inline_code_block line 8 at FOR over SELECT rows
LOCATION: report_invalid_token, json.c:1178

3) Expected output:

NOTICE: "abc"

I expected this output because json_populate_record works well with all json
values expect when the value is a simple string. I think this is an
incoherent behaviour (a string is a valid json value).

This bug affects also the jsonb_populate_record variant, as well as the
*_populate_recordset.

4) PostgreSQL version:

9.5alpha1 and 9.4 (haven't tested in 9.3)

5) Platform information:

Linux Ubuntu 14.4