fix: hoist function settings with memory units properly

f9ee1f7e introduced the hoisting of function settings as transaction-scoped
settings. However, this currently doesn't work with memory units, which are
case-sensitive according to the docs [1]. This removes the lowercasing of
values to make them work.

This is not added to the CHANGELOG, because this feature was not released, yet.

[1]: https://www.postgresql.org/docs/current/config-setting.html#CONFIG-SETTING-NAMES-VALUES

test/io/fixtures.sql

@@ -205,7 +205,7 @@ $$ language sql;
 
 create or replace function work_mem_test() returns text as $$
   select current_setting('work_mem',false);
-$$ language sql set work_mem = '6000';
+$$ language sql set work_mem = '6000kB';
 
 create or replace function multiple_func_settings_test() returns setof record as $$
   select current_setting('work_mem',false) as work_mem,