Fix an issue related to quoted identifier in Pgsql.

[Drup]

This should be safe, but it may breaks other people code as it makes all tables names case sensitive. I need it and I used it for quite a while, but I'm not sure it's mergeable.

[gasche]

You turn a %s into a "%s". I understand that %S wouldn't be quite right (OCaml's lexical conventions are not SQL's), but isn't there a potential problem with quotes needing to be escaped? I assume you rely on the parser rejecting them, but that's not good style.