prevent `time_zone_aware_attributes` test leak. follow-up to #12633.
cast json values on write to be consistent with reading from the db.

See also commit 5ac2341fab689344991b2a4817bd2bc8b3edac9d
Merge pull request #12643 from severin/pg_cast_json_on_write

cast json values on write to be consistent with reading from the db.