plpython: fix for improperly handled NULL arguments in prepared plans

The attached patch fixes a bug reported by Kevin Jacobs

When we create a function like so:

CREATE OR REPLACE FUNCTION jojo (timestamp) RETURNS varchar AS '
pln = plpy.prepare("UPDATE jtab SET jfld = $1",["timestamp"])
rcs = plpy.execute(pln,[args[0]])
return "YES"' LANGUAGE 'PLPYTHON';

SELECT jojo(now());
works fine but

SELECT jojo(NULL);
errors out with
NOTICE: plpython: in function __plpython_procedure_jojo_49818349:
plpy.Error: Unknown error in PLy_spi_execute_plan
ERROR: Bad timestamp external representation 'None'

NULL arguments were improperly converted to the string 'None' rather than being marked
as NULLs in the third argument to SPI_execp. The patch is against CVS HEAD.

--
Andrew Bosma <bosma@epigenomix.com>