BUG #16586: deduplicate_items=true can be configured for numeric indexes

The following bug has been logged on the website:

Bug reference:      16586
Logged by:          Matthias van de Meent
Email address:      matthias.vandemeent@cofano.nl
PostgreSQL version: 13beta3
Operating system:   Debian Stretch (9.13)
Description:

> CREATE INDEX numerical_index ON table USING btree ((num::numeric)) WITH
(deduplicate_items=true);
CREATE INDEX
> \d+ numerical_index
                    Index "public.numerical_index"
 Column |  Type   | Key? | Definition | Storage | Stats target 
--------+---------+------+------------+---------+--------------
 num    | numeric | yes  | num        | main    | 
btree, for table "public.table"
Options: deduplicate_items=true

There is no error for specifying the "deduplicate_items" -flag. As
deduplication is not supported for indexes with numeric type, I expected the
index creation statement to error.