Fix for changing parameter types with server prepared statements (was Re: setObject on PGInterval throws "Unknown Type null")

Oliver Jowett wrote:

> We currently break in the case where parameter types change between
> executions, but that's more easily fixable since we have all the
> necessary information already available.

I just fixed this in CVS, it wasn't as painful to do as I originally
thought. It should also work for the NULL-as-oid-0 case if we go back to
doing that -- any change in parameter type oids causes a reprepare.

-O