test case for custom PostgreSQL enum type.

This test currently outputs the following warning:

```
unknown OID: current_mood(3567879) (SELECT  "postgresql_enums".* FROM "postgresql_enums"   ORDER BY "postgresql_enums"."id" ASC LIMIT 1)
unknown OID: current_mood(3567879) (SELECT  "postgresql_enums".* FROM "postgresql_enums"  WHERE "postgresql_enums"."id" = $1 LIMIT 1)
```

We have an open PR to deal with this issue. It will dynamically
register the OID for enum columns. This test case is merely to exhibit
the current behavior of PostgreSQL enum columns.