From: Peter T Mount <patches@maidast.demon.co.uk>

This fixes a problem in ResultSet.getDate() when the column is NULL
(reported by Vincent Partington <Vincent.Partington@nmg.nl>)

And fixes a problem with Field's (ResultSet.getObject() was proving to be
slow as it repetedly send queries for oid -> name mapping - fixed by
creating a cache. (reported by Mario Ellebrecht <ellebrec@nads.de>)