Handle DBeaver's pg_namespace query casting to REGCLASS

[ediril]

Fixes: #29

DBeaver sends the following sql to get info regarding tables:

"SELECT n.oid, n.*, d.description 
FROM pg_catalog.pg_namespace AS n 
LEFT OUTER JOIN pg_catalog.pg_description AS d 
ON d.objoid = n.oid AND d.objsubid = 0 AND d.classoid = CAST('pg_namespace' AS REGCLASS) ORDER BY nspname"

DuckDB does not have a REGCLASS data type though, so it fails. This PR modifies the incoming sql to make it work on duckdb, by removing the cast to REGCLASS.

[ediril]

While this change makes the query work, it returns 3 mains because of duckdb's internal organization.

[jwills]

@ediril yeah going back and forth on the 3 mains thing-- that is technically how DuckDB works, right? So that's probably okay to do?