Test json rpc arguments

In particular, check buggy behavior around embedded quoted JSON
and Postgres versions, compare

https://www.postgresql.org/message-id/D6921B37-BD8E-4664-8D5F-DB3525765DCD%40vllmrt.net

This adds version-bounded tests for the handling of quoted JSON,
and a pending test that documents the assumption that Postgres >=10
intends to parse quoted JSON as a string (similar to how jsonb works
now).

test/fixtures/schema.sql

@@ -206,18 +206,28 @@ CREATE FUNCTION varied_arguments(
   date date,
   money money,
   enum enum_menagerie_type,
-  "integer" integer default 42
+  "integer" integer default 42,
+  json json default '{}',
+  jsonb jsonb default '{}'
 ) RETURNS text
     LANGUAGE sql
 AS $_$
   SELECT 'Hi'::text;
 $_$;
 
-COMMENT ON FUNCTION varied_arguments(double precision, character varying, boolean, date, money, enum_menagerie_type, integer) IS
+COMMENT ON FUNCTION varied_arguments(double precision, character varying, boolean, date, money, enum_menagerie_type, integer, json, jsonb) IS
 $_$An RPC function
 
 Just a test for RPC function arguments$_$;
 
+
+CREATE FUNCTION json_argument(arg json) RETURNS text
+
+LANGUAGE sql
+AS $_$
+  SELECT json_typeof(arg);
+$_$;
+
 --
 -- Name: jwt_test(); Type: FUNCTION; Schema: test; Owner: -
 --