BUG #14745: to_tsvector(regconfig, json[b]) is NOT immutable

The following bug has been logged on the website:

Bug reference: 14745
Logged by: Bob Jones
Email address: 007reader(at)gmail(dot)com
PostgreSQL version: 10beta2
Operating system: Debian in Docker container
Description:

According to https://www.postgresql.org/about/news/1763/ Beta 2 marked
to_tsvector(regconfig, json[b]) immutable, but it still doesn't work, at
least in the docker version of Beta 2. Here is the result:

create table test (j JSON, jb JSONB);
CREATE TABLE

create index j_ixd on test using gin(to_tsvector(j));
ERROR: functions in index expression must be marked IMMUTABLE

create index jb_ixd on test using gin(to_tsvector(jb));
ERROR: functions in index expression must be marked IMMUTABLE